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SECRE HVORCON/NOFORN

APR 1 4 2016
LeeAnn Flynn Hall, Clerk of Court

# United States Foreign Intelligence Surveillance Court of Rebiel

IN RE: CERTIFIED QUESTION OF LAW

Docket No. FISCR 16-01

Upon Certification for Review by the United States
Foreign Intelligence Surveillance Court

Decided:

MARC ZWILLINGER, ZwillGen PLLC, Washington, D.C.,
argued the case as court-appointed amicus curiae. With
him on the brief was JACOB A. SOMMER.

ADITYA BAMZAI, United States Department of Justice,
Washington, D.C., argued the case for the United States.
With him on the brief were JOHN P. CARLIN, STUART J.
EVANS, J. BRADFORD WIEGMANN, AND LISA M. FARABEE.

Before BRYSON, CABRANES, AND TALLMAN, Judges.
PER CURIAM.

The Foreign Intelligence Surveillance Court ("FISC")
certified this matter under 50 U.S.C. § 1803(j) for review
by this court. The FISC certified the following question to
us:

Whether an order issued under 50 U.S.C. § 1842 may
authorize the Government to obtain all post-cut-through

digits, subject to a prohibition on the affirmative investi-
gative use of any contents thereby acquired, when there is
no technology reasonably available to the Government
that would permit:

(1) a PR/TT [pen register/trap-and-trace] device to ac-
quire post-cut-through digits that are non-content
DRAS {dialing, routing, addressing, and signaling]
information, while not acquiring post-cut-through
digits that are contents of a communication; or

(2) the Government at the time it receives infor-
mation acquired by a PR/TT device, to discard
post-cut-through digits that are contents of a
communication, while retaining those digits that
are non-content DRAS information.

We have reviewed the record and considered briefs
from the government and from amicus curiae appointed
by the court under 50 U.S.C. § 1803(i) to present argu-
ment in this matter. We conclude that section 1842
authorizes, and the Fourth Amendment to the Constitu-
tion of the United States does not prohibit, an order of the
kind described in the FISC's certification. Read fairly and
as a whole, the governing statutes evince Congress's
understanding that pen registers and trap-and-trace
devices will, under some circumstances, inevitably collect
content information. Congress has addressed this diffi-
culty by requiring the government to minimize the inci-
dental collection of content through the employment of
such technological measures as are reasonably availa-
ble-not by barring entirely, as a form of prophylaxis, the
use of pen registers and trap-and-trace devices simply
because they might gather content incidentally.

Nor does an order authorizing such surveillance run
afoul of the Fourth Amendment's guarantee against
unreasonable searches and seizures. The warrant re-
quirement is generally a tolerable proxy for "reasonable-
ness" when the government is seeking to unearth

evidence of criminal wrongdoing, but it fails properly to
balance the interests at stake when the government is
instead seeking to preserve and protect the nation's
security from foreign threat. We therefore hold that
surveillance of this type may be constitutionally reasona-
ble even when it is not authorized by a probable-cause
warrant. We further hold, on the facts presented here,
that the order under review reasonably balances the
investigative needs of the government and the privacy
interests of the people.

On January 21, 2016, a judge of the FISC approved an
Application for Pen Register and Trap and Trace Device(s)
after finding that the application met the requirements
for a pen register/trap-and-trace authorization order
under the Foreign Intelligence Surveillance Act ("FISA").
The authorization provided for the installation and use of
pen register/trap-and-trace devices on a cellular telephone
number used by the subject of an ongoing investigation to
protect against clandestine intelligence activities, with
the assistance of the service provider for that number.1

As requested by the government, the court's order
granted "the authority to record and decode all post-cut-
through digits," as described in a memorandum filed by
the government with the FISC on August 17, 2009, in
connection with an earlier request for similar authoriza-
tion. The court's order further provided that the govern-

1 A pen register is a device or process that records or
decodes dialing signals transmitted from a telephone or
other wire or electronic communication instrument or
facility. A trap-and-trace device is a device or process
that captures incoming signals and therefore identifies
the originating number or source of an incoming wire or
electronic communication.

ment "shall not make any affirmative investigative use of
post-cut-through digits acquired through pen register
authorization that do not constitute call dialing, routing,
addressing or signaling information, unless separately
authorized by this Court." In a secondary order, the court
directed the service provider to furnish "all information,
facilities, or technical assistance necessary to accomplish
the installation and operation of the . . . device(s)."

"Post-cut-through digits" are numbers or characters
that are dialed after the call is initially connected or "cut
through." Frequently, those numbers are other telephone
numbers, as when a caller places a calling card, credit
card, or collect call by first dialing a carrier access number
and then, after the initial call is "cut through," dialing the
telephone number of the intended recipient. See U.S.
Telecom Ass'n v. FCC, 227 F.3d 450, 456, 462 (D.C. Cir.
2000); In re Application of the United States, 396 F. Supp.
2d 45, 48 (D. Mass. 2005). Both the first dialed number
(the carrier access number) and the second dialed number
(the intended recipient's number) constitute dialing
information.2 The initial dialed number, however, is
likely to be of little interest to investigators who are
seeking to determine what specific number the caller is

2 The statute that defines pen registers and trap-and-
trace devices for the purposes of this case refers to such
information as "dialing, routing, addressing, or signaling
information" utilized in the processing and transmitting
of wire or electronic communications, 18 U.S.C. § 3127(3),
(4). That phrase is sometimes represented by the acro-
nym DRAS. For simplicity, we will refer to that infor-
mation simply as "dialing information," but with the
understanding that the term is meant to include all four
categories of information set forth in section 18 U.S.C.
§ 3127, and to exclude what we shall refer to as "content
information."

calling. In such a situation, in order to discover what
number is being called, the investigators must be able to
intercept the post-cut-through digits.

In some instances, after a caller has dialed a tele-
phone number, the caller dials additional digits that do
not constitute dialing information, but instead constitute
a form of content information. For example, after dialing
a bank, the caller may be prompted to input a password, a
personal identification number, or a bank account num-
ber. Or, under certain circumstances, a customer may
enter a credit card number or a Social Security number by
dialing additional digits. That information is considered
content information. As the government acknowledges,
pen register orders do not target the interception and
decoding of such content information.3

The authorization granted by the FISC judge in this
case was consistent with prior FISC practice. Since at
least 2006, FISC judges have issued pen register/trap-
and-trace orders under 50 U.S.C. § 1842 that have au-
thorized the acquisition of all post-cut-through digits,
while generally prohibiting the use of those digits that do
not constitute dialing information.

3 The term "contents" has the same meaning in this
context as in the federal wiretapping statute, where it is
defined to mean "any information concerning the sub-
stance, purport, or meaning of [a wire, oral, or electronic]
communication." 18 U.S.C. § 2510(8); id. § 3127(1). A
different definition of "contents" is set forth at 50 U.S.C.
§ 1801(n). The definitions in section 1801, however, apply
to terms "[a]s used in this subchapter"-i.e., in 50 U.S.C.
§§ 1801-1812, the FISA subchapter on electronic surveil-
lance. That definition does not apply to "contents" for
purposes of the FISA subchapter on pen registers and
trap-and-trace devices, 50 U.S.C. §§ 1841-1846.

In the order certifying the question of law to this
court, the FISC judge set forth in detail the background of
the legal issue presented by the government's application.
The FISC judge also described the manner in which other
courts have dealt with this issue under the pen regis-
ter/trap-and-trace provisions of title 18 of the United
States Code, which govern the use of such devices in the
context of criminal investigations.

The FISC judge explained that the pen register/trap-
and-trace statutes provide that the information intercept-
ed by pen registers and trap-and-trace devices "shall not
include the contents of any communication." 18
U.S.C. § 3127(3), (4). A related section, however, states
that the government "shall use technology reasonably
available to it" that restricts the recording or decoding of
electronic or other impulses "so as not to include the
contents of any wire or electronic communications." Id.
§ 3121(c). In the past, the FISC judge explained, the
government has argued, and the FISC has accepted, that
in the absence of such reasonably available technology,
the government is permitted to obtain all post-cut-
through digits, so long as the investigative use of any
content information contained therein is prohibited.
Because there is not now and has not previously been any
known or reasonably available technology to segregate
dialing information from content information in post-cut-
through digits prior to the interception of those digits, the
government has contended that it is entitled to obtain
post-cut-through digits even when the acquisition of such
digits comes with some risk of intercepting content infor-
mation.

The FISC judge explained that the government's
interest in acquiring such digits is concretely presented in
this case. The subject of the investigation is suspected of
engaging in clandestine intelligence activities on behalf of
a foreign government, contrary to the interests of the
United States.

Using currently available technology, the government
cannot identify the foreign telephone number without
obtaining the entire set of post-cut-through digits.

Considering the competing privacy interests, the
FISC judge concluded that they are not great. Even
though some post-cut-through digits may constitute
content information, they "nonetheless involve a narrow
category of information from a subset of calls placed from
a targeted phone number." The intrusion, the judge
explained, is less than obtaining the full contents of calls
to or from a targeted number, and the intrusion is also
"mitigated by the prohibition on affirmative investigative
use" of the non-dialing information.

In view of the uniformity of the authorities holding
that post-cut-through digits may not be intercepted in the
parallel setting of criminal investigations, the FISC judge
concluded that the "disagreement between the FISC and
other courts provides reason to believe that consideration
of these issues by the [Foreign Intelligence Surveillance
Court of Review] would serve the interests of justice." See
50 U.S.C. § 1803(j). We find that it is appropriate for this
court to address the certified question.

The problem in this case is this: Under presently
available technology, there is no way for a pen register to
distinguish between dialing information and content
information contained in post-cut-through digits so that it
can be directed to intercept only the former and not the
latter.4 Therefore, in the case of a pen register order that

4 The amicus curiae argues that such technology al-
ready exists: the government can limit the collection of

ARADINMADONNAMADE

authorizes the interception of post-cut-through digits,
there is some risk that content information will be inter-
cepted along with dialing information. The question we
have been asked to decide is whether the statute that
authorizes the issuance of pen register orders for foreign
intelligence purposes permits courts to authorize the
interception of post-cut-through digits, even though there
is some risk that such digits might sometimes include
content information.

The statute that governs the use of pen registers and
trap-and-trace devices for foreign intelligence purposes is
title IV of FISA, 50 U.S.C. §§ 1841-46. That statute
provides that the government can obtain an order author-
izing the installation and use of a pen register or trap-
and-trace device upon a statutorily sufficient showing,
made either to a judge of the FISC or to a properly au-
thorized magistrate judge. Id. § 1842.

An application for a pen register or a trap-and-trace
device under section 1842 requires the approval of the
Attorney General or a designated attorney for the gov-
ernment. Id. § 1842(c). It also requires a certification by
the applicant that the information likely to be obtained "is
foreign intelligence information not concerning a United
States person or is relevant to an ongoing investigation to
protect against international terrorism or clandestine

digits to the first ten dialed digits. To be sure, that ap-
proach would exclude all content information, but at the
expense of excluding all dialing information that might be
present in post-cut-through digits, even in settings where
there is no reasonable likelihood of intercepting content
information. That is not a technological solution that
discriminates between dialing and content information, as
referred to in section 3121(c).

intelligence activities." Id. § 1842(c)(2). Finally, the
application must contain a "specific selection term" to be
used as the basis for the use of the pen register or the
trap-and-trace device. Id. § 1842(c)(3). A "specific selec-
tion term" is a term "that specifically identifies a person,
account, address, or personal device, or any other specific
identifier." Id. § 1841(4)(A)(i). It must be used to limit,
"to the greatest extent reasonably practicable, the scope of
information sought, consistent with the purpose for
seeking the use of the pen register or trap and trace
device." Id. § 1841(4)(A)(ii).

Section 1842(h)(1) of FISA provides that the Attorney
General "shall ensure that appropriate policies and pro-
cedures are in place to safeguard nonpublicly available
information concerning United States persons that is
collected through the use of a pen register or trap and
trace device installed under this section." Section
1842(h)(2) further provides that the FISC is not prohibit-
ed from imposing "additional privacy or minimization
procedures with regard to the installation or use of a pen
register or trap and trace device."

The definitional section of title IV of FISA, section
1841, provides that the terms pen register and trap-and-
trace device have the same meanings that are given to
those terms in section 3127 of the title 18. The definition
of pen register in section 3127 provides as follows, in
pertinent part:

[T]he term "pen register" means a device or pro-
cess which records or decodes dialing, routing, ad-
dressing, or signaling information transmitted by
an instrument or facility from which a wire or
electronic communication is transmitted, provid-
ed, however, that such information shall not in-
clude the contents of any communication .

18 U.S.C. § 3127(3). The definition of "trap and trace
device" in title 18 contains similar language:

NEVADAMADCONNAMADN

[T]he term "trap and trace device" means a device
or process which captures the incoming electronic
or other impulses which identify the originating
number or other dialing, routing, addressing, and
signaling information reasonably likely to identify
the source of a wire or electronic communication,
provided, however, that such information shall
not include the contents of any communication;

Id. § 3127(4).

The question whether title IV of FISA authorizes pen
register orders to collect post-cut-through digits turns on
the meaning of the definitional language in 18 U.S.C.
§ 3127(3), and in particular the "proviso" clause, which
reads as follows: "provided, however, that such infor-
mation shall not include the contents of any communica-
tion." It is clear that the statutory language is intended
to prohibit the use of pen registers for the purpose of
intercepting content communications, such as bank
account numbers, social security numbers, and personal
identification numbers. The statute expresses that intent
in an unusual way, however, by making the prohibition
against intercepting content information part of the
definition of "pen register."5

The most literal interpretation of section 3127(3), read
in isolation, leads to a problem. If a device ceases to be a
pen register whenever it intercepts post-cut-through
content information, it is impossible to know in advance

5 The statutory provisions that apply to trap-and-
trace devices are largely (but not entirely) parallel to the
provisions that apply to pen registers. Because our anal-
ysis of the legal issue presented in this case is the same
for both pen registers and trap-and-trace devices, we will
generally refer only to pen registers for simplicity.

whether the device is a pen register (and thus whether its
use may be authorized under title IV of FISA).

A pen register intercepts the digits that are dialed. It
does not distinguish between dialing information, on the
one hand, and dialed digits that constitute "the contents
of any communication," on the other. With currently
available technology, that distinction can be drawn only
after the information collected by the pen register has
been decoded. Defining a device as a pen register depend-
ing on the nature of the material it ultimately collects
thus poses a dilemma for courts that are asked to author-
ize the collection of dialing information, and in particular
post-cut-through digits. A court seeking to determine
whether to authorize a pen register application that
includes post-cut-through digits cannot know in advance
whether the device will intercept some content infor-
mation and therefore be ineligible for an authorization
order.

One approach to resolving that problem is to conclude
that if there is any chance that content information will
be intercepted, a pen register order that authorizes the
collection of post-cut-through digits may not be entered.
Adopting that theory, several courts have held that the
pen register statute does not authorize the collection of
any post-cut-through digits. See In re Application of the
United States, 622 F. Supp. 2d 411 (S.D. Tex. 2007); In re
Application of the United States, No. 6:06-mj-1130 (M.D.
Fla. June 20, 2006), affg In re Application of the United
States, No. 6:06-mj-1130 (May 23, 2006); In re Applica-
tions of the United States, 515 F. Supp. 2d 325 (E.D.N.Y.

2007); In re Application of the United States, 441 F. Supp.
2d 816 (S.D. Tex. 2006).6

The theory adopted by those courts might lead to the
conclusion that the collection of post-cut-through digits
may be authorized in circumstances in which the govern-

6 One of the courts that has addressed this issue has
concluded that all post-cut-through digits constitute
content information. In re Application of the United
States, No. 08 MC 0595, 2008 WL 5255815 (E.D.N.Y. Dec.
16, 2008). On that premise, the court declined to author-
ize the interception of post-cut-through digits. That
premise, however, is flawed, as it is well understood that
post-cut-through digits can include both dialing infor-
mation and content information, and that they may often
include only dialing information.

The amicus curiae argues that all post-cut-through
digits are content with respect to the service provider, and
that the interception of post-cut-through digits should
never be authorized. That argument is unconvincing, as
the definition of "contents" for purposes of pen registers is
"information concerning the substance, purport, or mean-
ing of [a wire, oral, or electronic] communication." 18
U.S.C. § 2510(8). That definition does not include dialing
information, whether viewed from the perspective of the
individual or the provider. The fact that the provider is
not the one who uses that information for dialing purpos-
es does not alter the fact that the information is dialing
information. The FCC made that point in its decision on
remand from U.S. Telecom Ass'n v. FCC, 227 F.3d 450
(D.C. Cir. 2000), cited by the amicus curiae. The FCC
explained that whether particular information is call-
identifying information has nothing to do with "whether a
carrier uses the dialed digits as part of its own call pro-
cessing." In re Communications Assistance for Law
Enforcement Act, 17 F.C.C.R. 6896 (2002).

SECRETWORCON/NOFORN

ment can assure the court that it is highly unlikely that
content information will be intercepted along with dialing
information. None of the above-cited decisions have
drawn that distinction, however. Rather, they have flatly
barred the government from relying on the pen register
statutes to intercept post-cut-through digits. See In re
Application of the United States, 622 F. Supp. 2d at 422
("If the Government has no means to exclude collecting
content when collecting post-cut-through dialed digits, the
Government may not obtain such information under the
Pen Register Statute."); In re Applications of the United
States, 515 F. Supp. 2d at 339 ("Until the Government can
separate PCTDD that do not contain content from those
that do, pen register authorization is insufficient for the
Government to obtain any PCTDD."); In re Application of
the United States, 441 F. Supp. 2d at 827 ("Post-cut-
through dialed digit contents ... are not available to law
enforcement under the Pen/Trap Statute."); In re Applica-
tion of the United States, No. 6:06-mj-1130, at 5 (M.D. Fla.
June 20, 2006) ("[T]his Court rejects the United States'
argument that it can obtain post-cut-through digits on the
lesser showing permitted by the pen register and trap-
and-trace statutes.").

We think the better approach is to interpret the
definitional language of section 3127(3) to mean that a
court may not authorize the use of a pen register to collect
content information, and that any content information
that is collected cannot be used for any investigative
purposes. Under that interpretation, a court can author-
ize the use of a pen register to collect post-cut-through
digits, as long as the collecting agency takes all reasona-
bly available steps to minimize the collection of content
information and is prohibited from making use of any
content information that may be collected.

We conclude that the latter interpretation of section
3127(3) is more in line with the statutory text and the

## NURLETOROUNNOFOREN

purpose the provision was intended to serve. In particu-
lar, we do not believe Congress intended to prohibit the
use of pen registers whenever there was any risk that the
intercepted digits would constitute content information.
To the contrary, we believe the best interpretation of the
related provisions of the pen register statutes is that
Congress understood that content information might
sometimes be intercepted by authorized pen registers, but
intended that steps should be taken to minimize that risk
to the extent reasonably possible. Both the text and the
legislative history of the pen register statutes support this
interpretation of section 3127(3).

It is clear from the text of the pen register provisions
in title 18, read as a whole, that Congress understood that
some content information might be intercepted in the
course of executing a valid pen register order. One of
those provisions is 18 U.S.C. § 3121(c). The statute
states:

(c) Limitation. A government agency author-
ized to install and use a pen register or trap and
trace device under this chapter or under State law
shall use technology reasonably available to it
that restricts the recording or decoding of elec-
tronic or other impulses to the dialing, routing,
addressing, and signaling information utilized in
the processing and transmitting of wire or elec-
tronic communications so as not to include the
contents of any wire or electronic communications.
18 U.S.C. § 3121(c).

That language requires the government to use "rea-
sonably available" technology to avoid recording content
information. But the prohibition is conditional, requiring
the government to use such restricting technology only if
it is "reasonably available." Thus, by requiring the use of

"technology reasonably available" to restrict recording
and decoding of intercepted information to dialing infor-
mation, Congress recognized that such technology might
not be available or might not achieve the objective with
perfect accuracy.

The plain import of the statutory language is that, ab-
sent such "reasonably available" technology, lawfully
authorized pen registers will sometimes intercept and
decode content information contained in dialed digits, in
addition to information regarding dialing information.
Thus, section 3121(c) strikes a compromise that allows
the government to obtain the dialing information to which
it is entitled, while requiring that all reasonably available
measures be taken to avoid or minimize the collection of
content information.

As the amicus curiae points out, section 3121(c) is not
incorporated by reference in title IV of FISA and therefore
does not directly apply to FISA pen register applications.
Nonetheless, it is important to our analysis here because
it provides guidance in determining how Congress intend-
ed courts to interpret the definitional provisions, sections
3127(3) and (4), which apply to both title 18 and title IV of
FISA. The argument that section 3121(c) is irrelevant to
FISA pen registers also ignores the body of law that
teaches that "where words are employed in a statute
which had at the time a well-known meaning at common
law or in the law of this country they are presumed to
have been used in that sense unless the context compels
to the contrary." Lorillard v. Pons, 434 U.S. 575, 583
(1978) (quoting Standard Oil v. United States, 221 U.S. 1,
59 (1911)).

Based on the legislative history of, and amendments
to, the criminal pen register statute, and Congress's
understanding of the developing technology, it can safely
be assumed that Congress-in incorporating the criminal
pen register definition into FISA-understood that it was

incorporating more than just the definition of a pen
register at section 3127. Indeed, the author of what
became section 3121(c), Senator Patrick Leahy, was quite
clear that the provision was necessary to address the
incidental collection of content under a pen register order.
147 Cong. Rec. 20,680 (2001) (statement of Sen. Patrick
Leahy). But at the same time Senator Leahy recognized
that the government's ability to avoid the collection of
content information was subject to the limitations of
"reasonably available technology." Id.

The amicus curiae takes the position that the defini-
tional language of section 3127(3)-"provided, however,
that such information shall not include the contents of
any communication"-plainly forecloses the conclusion
that a pen register may lawfully intercept content under
any circumstances. And some courts, likewise seizing on
the "provided" clause of section 3127(3), have dismissed
section 3121(c) as a mere "added precaution to ensure
that the Government does not use an authorized pen
register to collect contents." In re Application of the
United States, 622 F. Supp. 2d at 421.

We cannot agree with either position. Our duty is "to
construe statutes, not isolated provisions," and to properly
discharge that duty, "we must read the [statute's] words
in their context and with a view to their place in the
overall statutory scheme." King v. Burwell, 135 S. Ct.
2480, 2489 (2015). Of particular salience here, we are to
avoid interpreting one statutory provision in a manner
that would render another provision superfluous. Corley
v. United States, 556 U.S. 303, 314 (2009).

In focusing narrowly on section 3127(3) and giving
short shrift to the natural implication of section 3121(c),
the amicus curiae's plain-language argument and the
"added precaution" theory run afoul of these principles. If
section 3127(3) barred courts from authorizing the collec-
tion of post-cut-through digits, there would be no need for

technology to distinguish between dialing information and
content information. The need for technology to distin-
guish between the two types of information arises only if
the courts can authorize investigators to intercept signals
that can sometimes contain content. Because only post-
cut-through digits can contain content information, the
limitation of section 3121(e) must necessarily be directed
to post-cut-through digits. And because the limitation in
section 3121(c) is conditional, not absolute, the two provi-
sions can be read in harmony only by construing them to
permit the interception of post-cut-through digits under
appropriate circumstances.7

The background and development of the provisions of
title 18 that authorize the installation and use of pen
registers confirm our understanding of the statutory text
by shedding further light on the meaning of the pen

7 The amicus curiae contends that if the government's
argument were applied to Internet pen registers, the
government could collect information generated by a wide
variety of activities on the Internet, including searching,
uploading documents, and drafting emails.

Nonetheless, the amicus argues that the pro-
spect of such collections indicates that the government's
statutory construction must be wrong. We disagree.
Even assuming that the government's statutory theory
would apply in the same manner in that different techno-
logical setting, we would have to determine whether any
technology is reasonably available to excise content.
Moreover, the application of the government's theory in
that setting, if it had the consequences argued by amicus
curiae, might call for a different Fourth Amendment
balancing of interests.

Prior to 1986, there was no federal statute that gov-
erned the use of pen registers and trap-and-trace devices.
Title III of the Omnibus Crime Control and Safe Streets
Act of 1968, Pub. L. No. 90-351, 82 Stat. 197, dealt with
the interception of oral or wire communications that could
"be overheard and understood by the human ear." S. Rep.
No. 99-541, at 2 (1968). Title III was silent, however, as
to the use of pen registers or other devices that could
intercept non-content information.

In Smith v. Maryland, 442 U.S. 735 (1979), the Su-
preme Court held that the Fourth Amendment does not
apply to a pen register that simply monitors the digits
dialed on a party's telephone. The Court reasoned that
the calling party has voluntarily turned that dialing
information over to a third party and has assumed the
risk that the third party would turn that information over
to the government. Thus, the Court held that pen regis-
ters, unlike wiretaps that intercept conversations, could
be installed and operated without the need for a court
order.

In 1986, Congress changed that regime with the
enactment of the Electronic Communications Privacy Act
of 1986, Pub. L. No. 99-508, 100 Stat. 1848. That statute
added a provision authorizing the government to install
and use pen registers and trap-and-trace devices, but only
upon obtaining a court order. The showing required to
obtain such an order was less demanding than the proba-
ble cause showing required for a wiretap authorization,
however. For the installation and use of a pen register or
trap-and-trace device, the statute required only that the
government represent that the information being sought
was "relevant to an ongoing criminal investigation being
conducted" by the requester's agency. 18 U.S.C. § 3122(b)
(1988).

Eight years later, in the Communications Assistance
for Law Enforcement Act of 1994, Pub. L. No. 103-414,
108 Stat. 4279, Congress revisited the use of pen registers
and trap-and-trace devices. The legislative history of that
statute shows that Congress understood that pen regis-
ters were capable of intercepting content information in
the course of performing their authorized function of
intercepting dialing information.8 Congress's response to
that problem was to direct that the interception of content
incidental to the interception of dialing information was
to be minimized to the extent that it was technologically
feasible to do so.

In particular, Congress added the "limitation" provi-
sion, section 3121(c), to the pen register statutes. The
enacted version of section 3121(c) stated:

A government agency authorized to install and
use a pen register under this chapter or under
State law shall use technology reasonably availa-
ble to it that restricts the recording or decoding of
electronic or other impulses to the dialing and
signaling information utilized in call processing.

18 U.S.C. § 3121(c) (1994).

That provision recognized that pen registers were
capable of intercepting content information, Congress's
solution to that problem was to direct agencies using pen

8 The problem of pen registers intercepting "content"
or "transactional" information was discussed throughout
the Joint Hearing on the bill that became the 1994 stat-
ute. See Digital Telephony & Law Enforcement Access to
Advanced Telecomms. Techs. and Servs .: Joint Hearings
Before the Subcomm. on Tech. and the Law of the S.
Comm. on the Judiciary and the Subcomm. on Civil and
Constitutional Rights of the H. Comm. on the Judiciary,
103d Cong. 39-40, 50, 110-11, 114, 116, 158, 161 (1994).

registers to use technology that was "reasonably availa-
ble" to restrict the recording or decoding of content infor-
mation and limit the information obtained to "the dialing
and signaling information utilized in call processing." In
effect, Congress directed the agencies to do the best they
reasonably could to limit the interception of content
information, but it did not suggest that, in the absence of
such reasonably available technology, a pen register could
not be authorized if it posed the risk of intercepting
content information.

Both the House and Senate Reports on the 1994 Act
explained that the purpose of the amendment was not to
prohibit the use of pen registers, but to "requirell law
enforcement to use reasonably available technology to
minimize information obtained through pen registers." S.
Rep. No. 103-402, at 18 (1994); H.R. Rep. No. 103-827,
pt.1, at 17 (1994).9 In particular, the reports explained
that the new provision would require government agen-
cies "to use, when reasonably available, technology that
restricts the information captured by such device to the
dialing or signaling information necessary to direct or
process a call, excluding any further communication
conducted through the use of dialed digits that would
otherwise be captured." S. Rep. No. 103-402, at 31; H.R.
Rep. No. 103-827, pt. 1, at 32.

9 The term "minimization" has a familiar meaning in
the context of interceptions of electronic communications.
Section 2518(5) of title 18 directs that electronic surveil-
lance must "be conducted in such a way as to minimize
the interception of communications not otherwise subject
to interception." The requirement of minimization thus
contemplates that some unauthorized interception will
inevitably occur, but that the agency must take steps to
keep that interception to a minimum.

Senator Leahy, the principal sponsor of the legisla-
tion, used the same language when explaining the text of
the amendment during floor consideration of the legisla-
tion in the Senate. See 140 Cong. Rec. 20,451 (1994)
(statement of Sen. Patrick Leahy).

Accordingly, as matters stood after the 1994 legisla-
tion, the government could obtain authorization to use
pen registers, even though those devices might in some
instances intercept content information, as long as the
government used all technology that was reasonably
available to minimize the extent to which such content
information was intercepted and decoded.

Four years later, Congress amended FISA by adding
the pen register and trap-and-trace provisions of title IV,
50 U.S.C. § 1841 et seq. The new section 1841 provided
that the terms "pen register" and "trap and trace device"
were to "have the meanings given such terms in section
3127 of title 18." Pub. L. No. 105-272, 112 Stat. 2396,
§ 601 (1998).

Following the attacks against New York and Wash-
ington on September 11, 2001, Congress enacted the USA
PATRIOT Act of 2001, Pub. L. No. 107-56, 115 Stat. 272.
Among many other provisions, Congress modified portions
of the pen register/trap-and-trace statute. The changes
made at that time are at the heart of the issue before the
court today.

The principal change to the pen register/trap-and-
trace provisions was to make those provisions applicable
not just to telephony, but to all forms of wire and electron-
ic communications. In so doing, Congress made four
amendments that bear on the present issue.

First, Congress omitted the words "call processing"
and added the words "routing" and "addressing" to section
3121(c) to cover technologies other than telephony. Id.
§ 216(a).

Second, Congress modified section 3121(c) to state ex-
plicitly that the purpose of directing the government to
use "reasonably available" technology to limit the collec-
tion of certain electronic signals was "so as not to include
the contents of any wire or electronic communications."
Id.

Third, Congress amended the definition of "pen regis-
ter" by expanding the definition to include "dialing, rout-
ing, addressing, or signaling information transmitted by
an instrument or facility from which a wire or electronic
communication is transmitted." Id. § 216(c).

Fourth, Congress added the proviso in the definitions
of pen register and trap-and-trace device that read: "pro-
vided, however, that such information shall not include
the contents of any communication." Id.

The USA PATRIOT Act was enacted seven weeks
after the September 11, 2001, attacks, and in light of the
speed with which it was enacted, there is only limited
legislative history for the statute. The changes to sections
3121(c) and 3127 were added in the Senate. In the ab-
sence of a committee report, Senator Leahy, the chairman
of the Senate Judiciary Committee, presented a detailed
summary of the changes on the day before the Act was
passed. He explained that the language used in the pen
register and trap-and-trace statutes was intended "to
expressly exclude the use of pen-trap devices to intercept
'content' which is broadly defined in 18 U.S.C. 2510(8)."
147 Cong. Rec. 20,680 (2001) (statement of Sen. Patrick
Leahy). He added that the Act "requires the government
to use reasonably available technology that limits the
interceptions under the pen/trap device laws 'so as not to
include the contents of any wire or electronic communica-
tions." Id.

Importantly, Senator Leahy recognized that, notwith-
standing the statutory directive to use reasonably availa-
ble technology to avoid collecting content information, the

"pen/trap devices in use today collect 'content." Id. In
particular, he recognized the risk of collecting content
information from "[t]he impulses made after a phone call
is connected." Id. He explained that the amendment to
section 3121(c) was intended to underscore the need to
incentivize the development of better technology to limit
the interception of content information, particularly in
light of the fact that the USA PATRIOT Act made the pen
register provisions applicable to a wide array of modern
communications technologies, such as the Internet, and
not simply traditional telephone lines. See also H.R. Rep.
No. 107-236(I), at 52-53 (2001).

Senator Leahy stated that he was concerned that in
broadening the types of dialing information that could be
intercepted to include routing and addressing infor-
mation, Congress might be misunderstood as authorizing
the interception of content information. He said that to
address that issue, he had favored including definitions of
those terms in the 2001 statute, but that the administra-
tion had objected. Instead, to address his concerns, the
administration agreed to include the references to content
information in sections 3121(c) and 3127(3) and (4).

Senator Leahy also noted that, in light of the known
risk of collecting content information from post-cut-
through digits, he would have preferred a requirement of
somewhat heightened judicial review for pen register and
trap-and-trace applications. But in the absence of such a
requirement, he acknowledged that the statute continued
to require only that the government "use reasonably
available technology" to limit the collection of content
information.

Senator Leahy's comments make clear that the new
language added in the 2001 statute was intended to avoid
expanding the type of information that could be intercept-
ed, not to narrow it. In particular, nothing in his com-
ments, or elsewhere in the legislative history, suggests

## DEVRE IT VRVONTOFUEN

that, in the absence of an effective technological solution,
the amendments to the pen register/trap-and-trace stat-
utes were intended to prohibit the collection of dialing
information simply because there was some risk that
content information might incidentally be collected as
well.

Analysis of the sequence of pertinent statutes leads us
to conclude that Congress recognized, from as early as
1994, that judicial authorization to collect post-cut-
through digits posed the risk that some content infor-
mation would be intercepted. But Congress chose to deal
with that risk by requiring the government to use reason-
ably available technology to minimize the extent to which
such content information was collected. It could have
dealt with that risk by preventing the collection of post-
cut-through digits altogether, but it did not.

We therefore conclude that a close analysis of the
statutes that have authorized pen register orders starting
in 1986 does not support the view that Congress sought to
prohibit any authorized collection of dialing information
whenever it posed some risk of additionally collecting
content information. What Congress elected was a course
of minimization, principally through the use of "reasona-
bly available technology."

## III

Our analysis of the pen register statutes requires us
to consider whether those statutes, if construed to author-
ize the interception of post-cut-through digits, would run
afoul of the Fourth Amendment.

As noted above, the Supreme Court in Smith v. Mary-
land held that the use of a pen register to collect the
numbers dialed on a target telephone does not constitute
a "search" for Fourth Amendment purposes. The Smith
case, however, involved the use of a pen register to obtain
dialing information only; no content information was at

issue in that case, in the form of post-cut-through digits or
otherwise.

It may be that if a pen register interception were di-
rected at the acquisition and use of content information, it
would be unlawful in the absence of a court order issued
on a showing of probable cause. In the context of criminal
investigations, that would certainly be the case for the
interception of conversations through electronic surveil-
lance, Berger v. New York, 388 U.S. 41 (1967), and it has
been held that probable cause is required to authorize the
disclosure and use of content information in email com-
munications, see Warshak v. United States, 490 F.3d 455
(6th Cir. 2007), vacated, 532 F.3d 521 (6th Cir. 2008) (en
banc). The same rule might apply to the use of a pen
register for the purpose of intercepting content infor-
mation.

But the FISC judge's authorization order for post-cut-
through digits does not target content information; it
targets dialing information. If content information is
collected at all, the collection of that information is inci-
dental, and the FISC judge's authorization order directs
that no investigative use be made of that information (at
least in the absence of a further order from the court).
The constitutional issue, therefore, is not whether a
probable cause warrant is required to use a pen register
to obtain content information for investigative purposes.
Rather, the question is whether the risk of incidental
collection of content information renders the collection of
dialing information in post-cut-through digits unreasona-
ble in the absence of a probable cause warrant, even when
the content information will not be used for any purpose.
We think the answer to that question is no.

The touchstone of the Fourth Amendment is reasona-
bleness. Riley v. California, 134 S. Ct. 2473, 2482 (2014);
United States v. Knights, 534 U.S. 112, 118 (2001); see
also Vernonia Sch. Dist. 47J v. Acton, 515 U.S. 646, 652

DHVWWWTINKOVNROTURN

## SECRETZURCONINUFURN

(1995); In re Sealed Case, 310 F.3d 717, 742 (F.I.S.C.R.
2002). In determining the reasonableness of particular
governmental action, the court must assess, "on the one
hand, the degree to which it intrudes upon an individual's
privacy and, on the other, the degree to which it is needed
for the promotion of legitimate governmental interests."
Wyoming v. Houghton, 526 U.S. 295, 300 (1999); see also
Tennessee v. Garner, 471 U.S. 1, 8 (1985); United States v.
Place, 462 U.S. 696, 703 (1983); In re Directives Pursuant
to Section 105B of the Foreign Intelligence Surveillance
Act, 551 F.3d 1004, 1012 (F.I.S.C.R. 2008).

When law enforcement officials undertake a search to
uncover evidence of criminal wrongdoing, the familiar
requirement of a probable-cause warrant generally
achieves an acceptable balance between the investigative
needs of the government and the privacy interests of the
people. See Vernonia Sch. Dist. 47J, 515 U.S. at 653. But
it has long been recognized that some searches occur in
the service of "special needs, beyond the normal need for
law enforcement," and that, when it comes to intrusions of
this kind, the warrant requirement is sometimes a poor
proxy for the textual command of reasonableness. Id.

We conclude that, in the circumstances presented
here, the incidental collection of content information
during the collection of post-cut-through digits-assuming
it constitutes a search in the first place-is constitutional-
ly reasonable, even when done without a probable-cause
warrant.

The idea that official intrusions calculated to preserve
the nation's security against foreign threat might require
special constitutional treatment is not a new one. In Katz
v. United States, the first page in the modern chapter of
our search-and-seizure jurisprudence, the Supreme Court
paused to observe that the Fourth Amendment's usual
strictures might require adjustment "in a situation involv-
ing national security." 389 U.S. 347, 358 n.23 (1967).

Five years later, in United States v. United States District
Court (Keith), the Court rejected the argument that no
warrant need be obtained whenever the government
engages in domestic surveillance related to "internal
security matters." 407 U.S. 297, 299 (1972). But it took
care to emphasize that Keith "involve[d] only the domestic
aspects of national security," not any "issues which may
be involved with respect to activities of foreign powers or
their agents," id. at 321-22, and it noted "the view that
warrantless surveillance, though impermissible in domes-
tic security cases, may be constitutional where foreign
powers are involved," id. at 322 n.20.

Consistent with this counsel, in the decade following
Keith, a number of federal appeals courts recognized a
"foreign intelligence" exception to the warrant require-
ment. See United States v. Truong Dinh Hung, 629 F.2d
908, 912-16 (4th Cir. 1980); United States v. Buck, 548
F.2d 871, 875 (9th Cir. 1977); United States v. Butenko,
494 F.2d 593, 604-06 (3d Cir. 1974) (en banc); United
States v. Brown, 484 F.2d 418, 426 (5th Cir. 1973). But
see Zweibon v. Mitchell, 516 F.2d 594, 633-51 (D.C. Cir.
1975) (en banc) (plurality opinion) (suggesting, in dictum,
that no such exception exists).10

10 The dictum in Zweibon was not joined by a majori-
ty of the court. As the D.C. Circuit has recognized in
subsequent cases, the Zweibon court barred "warrantless
electronic surveillance of persons not suspected of collabo-
ration with foreign interests adverse to this country," but
"there was no opinion of the court on the question of
warrantless electronic surveillance of collaborators or
suspected collaborators of foreign interests." Halperin v.
Helms, 690 F.2d 977, 1000 n.82 (D.C. Cir. 1982); see also
Ellsberg v. Mitchell, 709 F.2d 31, 66 n.63 (D.C. Cir. 1983);
United States v. Belfield, 692 F.2d 141, 145 (D.C. Cir.

## WHOWEETTURWVUNROFORN

Truong is illustrative. In that case, the FBI became
aware that David Truong, a Vietnamese citizen living in
the United States, was obtaining classified papers from a
source within the federal government and endeavoring to
send them to Vietnamese officials in Paris. 629 F.2d at
911-12. With the approval of the Attorney General, but
no judicial warrant, Truong's phone was tapped and his
apartment "bugged." Id. at 912. He challenged the
admission at trial of evidence obtained through this
warrantless surveillance, but the district court admitted
much of it, and the Fourth Circuit affirmed. The appeals
court observed that, in the area of foreign intelligence, the
needs of the executive are particularly "compelling," and
that a warrant requirement would cripple the govern-
ment's ability to counter threats from abroad with the
needed "stealth, speed, and secrecy." Id. at 913. Accord-
ingly, it held that a search may be constitutionally rea-
sonable, notwithstanding the absence of prior judicial
authorization, when "the object of the search or the sur-
veillance is a foreign power, its agent or its collaborators,"
and "the search is conducted primarily for foreign intelli-
gence reasons." Id. at 915 (emphasis supplied) (internal
quotation marks omitted).11

More recently, this court both acknowledged the ex-
istence of a foreign-intelligence exception to the warrant
requirement and explained its doctrinal underpinnings.
See In re Directives, 551 F.3d at 1010-12. In In re Direc-
tives, we noted that in so-called "special needs" cases, the
Supreme Court has "excused compliance with the War-

1983); Chagnon v. Bell, 642 F.2d 1248, 1259 (D.C. Cir.
1980).

11
Consistent with this "primary purpose" require-
ment, the court affirmed the exclusion of evidence gleaned
after the date when the government had "begun to as-
semble a criminal prosecution." Truong, F.2d at 916.

rant Clause when the purpose behind the government
action went beyond routine law enforcement and insisting
upon a warrant would materially interfere with the
accomplishment of that purpose." Id. at 1010. The gov-
ernment may, for instance, engage in certain warrantless
intrusions when it acts as educator; blind adherence to
the Warrant Clause in the public schools "would unduly
interfere with the maintenance of the swift and informal
disciplinary procedures that are needed, and . . . undercut
the substantial need of teachers and administrators for
freedom to maintain order." Vernonia Sch. Dist. 47J, 515
U.S. at 653. So too may it maintain sobriety checkpoints
at which vehicles are stopped (and drivers thereby seized)
without suspicion, in the interest of curbing the harms
occasioned by drunk driving. Michigan Dep't of State
Police v. Sitz, 496 U.S. 444, 450-51 (1990).

We recognized in In re Directives that when the gov-
ernment engages in foreign intelligence surveillance-no
less than when it acts to maintain discipline in the
schools or operates sobriety checkpoints-its needs go
beyond "any garden-variety law enforcement objective,"
and its objectives would be seriously hampered by the
requirement of a warrant. In re Directives, 551 F.3d at
1011. Collecting foreign intelligence with an eye toward
safeguarding the nation's security serves an interest-a
"particularly intense" interest-different from the gov-
ernment's interest in the workaday enforcement of the
criminal law.12 And if the government were constrained

12 In discussing the importance of the government's
interest in preserving and protecting national security, we
criticized Truong's primary-purpose requirement as
"unstable, unrealistic and confusing." In re Directives,
551 F.3d at 1011 (internal quotation marks omitted). “A
surveillance with a foreign intelligence purpose," we
observed, "often will have some ancillary criminal-law

to obtain a warrant before undertaking any foreign intel-
ligence gathering that constituted a search, its "ability to
collect time-sensitive information" would be "hinder[ed]"
and "the vital national security interests at stake" imped-
ed. Id. We thus held that the Fourth Amendment does
not require a probable-cause warrant "when surveillance
is conducted to obtain foreign intelligence for national
security purposes and is directed against foreign powers
or agents of foreign powers reasonably believed to be
located outside the United States." Id. at 1012.

In re Directives virtually controls this case. The rele-
vant statute at issue in this case authorizes the use of a
pen register "to obtain foreign intelligence information . . .
to protect against ... clandestine intelligence activities."
50 U.S.C. § 1842(a)(1). Pursuant to that statute, the
government seeks to monitor the dealings of a person,
currently in the United States, who is suspected of collect-
ing intelligence in the service of a foreign power. The
purpose of the proposed monitoring is the preservation of
national security. Few government interests are of a
higher order. The interest at stake is no less-and may
even be greater-for the foreign agent's being present in
this country. And were we to insist on a showing of
probable cause and the issuance of a judicial warrant in
this setting, we would impede the Executive's ability to.
bring to bear against the threat those faculties-"stealth,
speed, and secrecy," Truong, 629 F.2d at 913-needed to
secure the nation's well-being in this most fundamental
and sensitive of government endeavors.

We thus conclude that when the government, acting
pursuant to a program of surveillance involving a legiti-

purpose." Id. We therefore concluded that the more
sensible requirement was that the "programmatic pur-
pose" of the intelligence-gathering "involvell some legiti-
mate objective beyond ordinary crime control." Id.

## DEUNUITURCONINOFORN

mate objective that goes beyond everyday crime control,
seeks to use a pen register directed at a person located in
the United States who is reasonably believed to be en-
gaged in clandestine intelligence activities on behalf of a
foreign government, it may do so without obtaining a
probable-cause warrant even if its monitoring of post-cut-
through digits constitutes a search under the Fourth
Amendment.

This is not to say, of course, that the Fourth Amend-
ment has no role to play in such cases. It is only to say
that, in this context, the warrant requirement is ill-suited
to gauge what is reasonable. The textual command of
reasonableness-"the ultimate touchstone of the Fourth
Amendment," Riley, 134 S. Ct. at 2482-still governs.
Indeed, it retains its whole force.

We now turn to the question of reasonableness, a
question that requires us to balance against the degree of
the government's intrusion on individual privacy the
degree to which that intrusion furthers the government's
legitimate interests. Houghton, 526 U.S. at 300. In the
circumstances presented here, the scale tips in the gov-
ernment's favor. The search, assuming it is one, is rea-
sonable. In particular, the factors that render the search
reasonable are (1) the paramount interest in investigating
possible threats to national security; (2) the investigative
importance of having access to the dialing information
provided by post-cut-through digits, (3) the incidental
nature of the collection of content information from post-
cut-through digits, (4) the relatively slight intrusion on
privacy entailed by the acquisition of post-cut-through
digits, (5) the prohibition against the use of any content
information obtained from the pen register or trap-and-
trace device, (6) the steps taken by the government to
minimize the dissemination of post-cut-through digits;
and (7) the fact that FISA pen register interceptions are
conducted only with the approval and under the supervi-

sion of a neutral magistrate, in this case a FISC judge.
We discuss each of those factors in more detail below.

First, the Supreme Court has stated that "no govern-
mental interest is more compelling" than national securi-
ty. Haig v. Agee, 453 U.S. 280, 307 (1981); see In re
Directives, 551 F.3d at 1012 (the governmental interest in
national security "is of the highest order of magnitude");
In re Terrorist Bombings of U.S. Embassies, 552 F.3d 157,
174 (2d Cir. 2008). Thus, the government's investigative
interest in cases arising under FISA is at the highest level
and weighs heavily in the constitutional balancing pro-
cess.

Second, as the facts of this case demonstrate, the dial-
ing information in post-cut-through digits may be of
critical investigative importance in certain cases in which
pen register authorization is sought. If the subject of a
pen register uses a calling service, a pen register that
does not collect post-cut-through digits will disclose no
information at all about the ultimate destination of the
call. Because subjects of national security investigations
seek to avoid detection of their activities, the loss of access
to post-cut-through digits is likely to substantially under-
cut the value of a pen register in a significant number of
cases.

Third, a pen register authorized in a FISA investiga-
tion is targeted at dialing information; the collection of
any content information from post-cut-through digits is
incidental to the purpose of the pen register. The inci-
dental collection of constitutionally protected material
does not render the authorized collection of unprotected
material unlawful. See In re Directives, 551 F.3d at 1015
(citing United States v. Kahn, 415 U.S. 143 (1974), and
United States v. Schwartz, 535 F.2d 160 (2d Cir. 1976)
("Incidental collections occurring as a result of constitu-
tionally permissible acquisitions do not render those
acquisitions unlawful.")).

The application of that rule to searches of documents
is particularly instructive here. The Supreme Court
recognized in Andresen v. Maryland, 427 U.S. 463, 482
n.11 (1976), that "[i]n searches for papers, it is certain
that some innocuous documents will be examined, at least
cursorily, in order to determine whether they are, in fact,
among those papers authorized to be seized." The inci-
dental examination of such documents to determine
whether they are subject to authorized seizure is analo-
gous to the examination of post-cut-through digits to
determine if they contain content information; once it is
determined that particular post-cut-through digits con-
tain content information, that information is excluded
from any investigative use.

Fourth, the content information found in some post-
cut-through digits is likely to be of marginal privacy
value. As the FISC judge explained in the certification
order, post-cut-through digits that constitute contents
"involve a narrow category of information from a subset of
calls placed from a targeted phone number" and thus
represent "a lesser intrusion than, for example, obtaining
the full contents of all calls to or from a targeted phone
number." For that reason, in balancing the seriousness of
the invasion of the individual's personal privacy against
the importance of the government's interest, the degree of
the intrusion resulting from collecting post-cut-through
digits will typically be modest.

Fifth, as the FISC judge's authorization order makes
clear (and is uniformly reflected in FISC pen regis-
ter/trap-and-trace authorization orders), any content
information that is collected as part of the interception of
post-cut-through digits may not be used for any investiga-
tive purpose, absent an order from the court.13 That

13 The government advises us that in the course of its
pen register investigations, no such order has ever been

prohibition on use protects against the risk that an inves-
tigative agency might seek to obtain authorization to
intercept post-cut-through digits in order to obtain access
to the content information contained therein.

Sixth, minimization procedures are available, and are
regularly employed, to limit the extent to which content
information that is incidentally intercepted during the
collection of post-cut-through digits is made available to,
or used and disseminated by, government agents.

The Department of Justice has taken several steps to
minimize access to post-cut-through digits and reduce the
risk that content information will be intercepted or dis-
closed. The prohibition against targeting or using content
information obtained from post-cut-through digits was set
forth in a 2002 memorandum of the Deputy Attorney
General, and the FBI's field offices have been instructed
to implement procedures to ensure compliance with the
policies in that memorandum. See Memorandum from
Larry D. Thompson, Deputy U.S. Attorney Gen., Avoiding
Collection and Investigative Use of "Content" in the
Operation of Pen Registers and Trap and Trace Devices
(May 24, 2002).

Among those procedures is a measure that requires
masking post-cut-through digits in investigative file
materials. Only an analyst who has undergone special
training may unmask the post-cut-through digits, and
only after providing justification for doing so. Record on
Appeal, Tab 3, at 17-20. In some circumstances, depend-
ing on the nature of the subscriber to the telephone that
was initially contacted, even an analyst may not examine
post-cut-through digits. For example, if the initial con-

granted; in fact, the government has never even sought
such an order. See also Record on Appeal, Certification at
2 n.1.

## MINUNETTORVONRUFUIN

nection is to a financial institution, an analyst may not
examine any post-cut-through digits because there is
reason to believe that post-cut-through digits may contain
content.

Minimization measures have been recognized as
important to the lawfulness of investigative procedures in
various settings. Most significantly, federal wiretap law
recognizes that some conversations that were not intend-
ed to be intercepted will inevitably be overheard. The
answer given by Congress and endorsed by the courts is to
require minimization of such intrusions to the extent
reasonably practicable. See Scott v. United States, 436
U.S. 128, 139-43 (1978); Drimal v. Tai, 786 F.3d 219, 223-
24 (2d Cir. 2015); United States v. Glover, 681 F.3d 411,
420-21 (D.C. Cir. 2012).

The Supreme Court has applied the same principle to
document searches, emphasizing the importance of mini-
mization in both settings. See Andresen, 427 U.S. at 482
n.11 ("In both kinds of searches [searches of conversations
and searches of documents], responsible officials, includ-
ing judicial officials, must take care to assure that they
are conducted in a manner that minimizes unwarranted
intrusions upon privacy."). And in other Fourth Amend-
ment contexts as well, the Supreme Court has empha-
sized the importance of minimization steps employed to
reduce the intrusiveness of the invasion in question. See,
e.g., Maryland v. King, 133 S. Ct. 1958, 1979-80 (2013)
(acquisition of arrestees' DNA less intrusive because
authorized for use only for limited purpose of identifica-
tion); Bd. of Educ. of Indep. Sch. Dist. No. 92 of Pottawat-
omie Cty. v. Earls, 536 U.S. 822, 832-33 (2002) (school
drug testing program less intrusive because results kept
in confidential files and used for only limited purposes);
Vernonia School Dist. 47J, 515 U.S. at 658 (school drug
testing program less intrusive because of limited purpose
of tests and limited dissemination of results).

Finally, an important aspect of the use of pen regis-
ters in FISA investigations is the role played by FISC
judges in authorizing and supervising pen register inter-
ceptions. Although the court does not require a showing
of probable cause to authorize pen register interceptions,
it is responsible for supervising the execution of pen
register orders. As noted above, title IV of FISA contains
a provision authorizing FISC judges "to impose additional
privacy or minimization procedures with regard to the
installation or use of a pen register or trap and trace
device." 50 U.S.C. § 1842(h)(2).

In appropriate circumstances, FISC judges can use
that authority to ensure that the interception of content
information through the collection of post-cut-through
digits is kept to a minimum, consistent with the govern-
ment's right to intercept dialing information. Besides
requiring that the government use all reasonably availa-
ble technology to minimize or eliminate the collection of
content information, FISC judges can insist that the
government assess the risk of intercepting content infor-
mation in particular cases and can deny authorization for
post-cut-through digits (or impose further restrictions)
when that risk is deemed to be unacceptably high as, for
example, in the case of a request to renew an application
for a pen register that has previously intercepted a sub-
stantial amount of content information.14

The judicial scrutiny of pen register applications and
the supervision of the execution of pen register orders
further reduces the risk that such measures will be em-

14 In addition to the statutory authorization for the
imposition of minimization procedures, FISA contains a
suppression remedy that is available if information from
pen registers or trap-and-trace devices was unlawfully
acquired or if the devices were not operated in conformity
with the authorizing order. 50 U.S.C. § 1845(e)(1).

ployed under circumstances, or in a manner, that unrea-
sonably intrudes on individuals' privacy interests.

In sum, we hold that the request in this case for
authorization to intercept post-cut-through digits satisfies
the reasonableness standard of the Fourth Amendment.
Put another way, the Constitution does not go so far as to
impose an across-the-board prohibition on the collection of
dialing information in the absence of probable cause,
simply because of the risk that some content information
will be incidentally intercepted as well.

We conclude that Congress intended to minimize the
collection of content information by insisting that reason-
ably available technology be used to segregate dialing
information from content information. The government
represents-and we have no reason to doubt-that no
such technology is currently reasonably available. In that
circumstance, we conclude that the government is not
barred from using pen registers and trap-and-trace devic-
es to intercept post-cut-through digits because of the risk
that the use of those devices might, in some instances,
intercept digits that turn out to constitute content infor-
mation.

It is true that Congress intended to bar courts from
authorizing the use of pen registers that target content
information. That is not to say, however, that Congress
intended to prevent the use of pen registers for the legiti-
mate purpose of obtaining dialing information simply
because there was some risk that the pen registers would
inadvertently intercept content information in the course
of an authorized and lawful interception.

For the reasons set forth above, we answer the certi-
fied question in this matter as follows: the FISC may
authorize the collection and decoding of post-cut-through
digits as long as the government is prohibited from mak-

GRADAMARCANINAWARN

ing investigative or evidentiary use of any content infor-
mation contained in that material, and as long as the
court directs that appropriate procedures be used to
minimize the collection of content information, including
the use of any reasonably available technology that may
be developed to restrict the recording and decoding of pen
register or trap-and-trace information to dialing infor-
mation.

OPINION

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