In re Certified Question of Law
Thе Foreign Intelligence Surveillance Court (“FISC”) certified this matter under
Whether an order issued under
(1) a PR/TT [pen register/trap-and-trace] device to acquire 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 information 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.
Nor does an order authorizing such surveillance run afoul of the Fourth Amendment’s guarantee against unreasonable searches and seizures. The warrant requirement is generally a tolerable proxy for “reasonableness” 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 reasonable 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.
I
On January 21, 2016, a judge of the FISC approved an Application for Pеn 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.
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 authorization. The court’s order further provided that the government “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-сut-through digits” are numbers or characters that are dialed after the call
In some instances, after a caller has dialed a telephone 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 number. 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.
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
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 register/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 intercepted by pen registers and trap-and-trace devices “shall not include the contents of any communication.”
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, [redacted] 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
II
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 nоt the latter.
A
The statute that governs the use of pen registers and trap-and-trace devices for foreign intelligence purposes is title IV of FISA,
An application for a pen register or a trap-and-trace device under
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
[T]he term “pen register” means a device or process which records or decodes dialing, routing, addressing, or signaling information transmitted by an instrument or facility from which a wire or electronic communication is transmitted, provided, however, that such information shall not include the contents of any communication....
[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;
B
The question whether title IV of FISA authorizes pen register orders to
The most literal interpretation of
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 depending on the nature of the material it ultimately collects thus poses a dilemma for courts that are asked to authorize the collection of dialing infоrmation, 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 information 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,
We think the better approach is to interpret the definitional language of
We conclude that the latter interpretation of
1
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
(c) Limitation. A government agency authorized 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 electronic or other impulses to the dialing, routing, addressing, and signaling information utilized in the processing and transmitting of wire or electronic communications so as not to include the contents of any wire or electronic communications.
That language requires the government to use “reasonably 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 information, 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, absent 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,
As the amicus curiae points out,
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
The amicus curiae takes the position that the definitional- language of
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, — U.S. -,
In focusing narrowly on
2
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 register statutes in general, and
Prior to 1986, there was no federal statute that governed 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,
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 probable 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.
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 рen registers were capable of intercepting content information in the course of performing their authorized function of intercepting dialing information.
In particular, Congress added the “limitation” provision,
A government agency authorized to install and use a pen register under this chapter or under State law shall use technology reasonably available to it that restricts the recording or decoding of electronic or other impulses to the dialing and signaling information utilized in call processing.
That provision recognized that pen registers were capable of intercepting content information. Congress’s solution to that problem was to direct agencies using pen registers to use technology that was “reasonably available” to restrict the recording or decoding of content information 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 absеnce 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 “require[] 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-
Senator Leahy, the principal sponsor of the legislation, used the same language when explaining the text of the amendment during floor consideration of the legislation in the Senate. See 140 Cong. Rec. 20,451 (1994) (statement of Sen. Patrick Leahy).
Accordingly, as matters stood after the 1994 legislation, the government could obtain authorization to use pеn 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,
Following the attacks against New York and Washington 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 electronic 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
Second, Congress modified
Third, Congress amended the definition of “pen register” by expanding the definition to include “dialing, routing, 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: “provided, however, that such information shall not include the contents of any communication.” Id.
Importantly, Senator Leahy recognized that, notwithstanding the statutory directive to use reasonably available 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
Senator Leahy stated that he was concerned that in broadening the types of dialing information that could be intercepted to include routing and addressing information, Congress might bе 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 administration had objected. Instead, to address his concerns, the administration agreed to include the references to content information in
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 intercepted, not to narrow it. In particular, nothing in his comments, or elsewhere in the legislative history, suggests that, in the absence of an effective technological solution, the amendments to the pen register/trap-and-trace statutes 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 information would be inter
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 “reasonably available technology”
III
Our analysis of the pen register statutes requires us to consider whether those statutes, if construed to authorize the interception of post-cut-through digits, would run afoul of the Fourth Amendment.
As noted above, the Supreme Court in Smith v. Maryland 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 directed 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 surveillance, Berger v. New York,
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 incidental, and the FISC judge’s authorization order directs thаt 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 unreasonable 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 reasonableness. Riley v. California, — U.S.-,
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 nеeds of the government and the privacy interests of the people. See Vemonia Sch. Dist. 47J,
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 constitutionally 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 modem 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 involving national security.”
Consistent with this counsel, in the decade following Keith, a number of federal appeals courts recognized a “foreign intelligence” exception to the warrant requirement. See United States v. Truong Dinh Hung,
More recently, this court both acknowledged the existence of a foreign-intelligence exception to the warrant requirement and explained its doctrinal underpinnings. See In re Directives,
We recognized in In re Directives that when the government 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,
In re Directives virtually controls this case. The relevant statute at issue in this case authorizes the use of a pen register “to obtain foreign intelligence information ... to protect against ... clandestine intelligence activities.”
We thus conclude that when the government, acting pursuant to a program of surveillance involving a legitimate 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 engaged 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 Amendment 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,
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,
First, the Supreme Court has stated that “no governmental interest is more compelling” than national security. Haig v. Agee,
Second, as the facts of this case demonstrate, the dialing 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. Becаuse 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 undercut the value of a pen register in a significant number of cases.
Third, a pen register authorized in a FISA investigation 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 incidental collection of constitutionally protected material does not render the authorized collection of unprotected material unlawful. See In re Directives,
The application of that rule to searches of documents is particularly instructive here. The Supreme Court recognized in Andresen v. Maryland,
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
Fifth, as the FISC judge’s authorization order makes clear (and is uniformly reflected in FISC pen register/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 investigative purpose, absent an order from the court.
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 disclosed. 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 fоr doing so. Record on Appeal, Tab 3, at 17-20. In some circumstances, depending on the nature of the subscriber to the telephone that was initially contacted, even an analyst may riot examine post-cut-through digits. For example, if the initial connection 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 intended 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,
The Supreme Court has applied the same principle to document searches, emphasizing the importance of minimization in both settings. See Andresen,
Finally, an important aspect of the use of pen registers in FISA investigations is the role played by FISC judges in authorizing and supervising pen register interceptions. 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.”
In appropriate circumstances, FISC judges can use that authority to ensure that the interception of content information through the colleсtion of post-cut-through digits is kept to a minimum, consistent with the government’s right to intercept dialing information. Besides requiring that the government use all reasonably available technology to minimize or eliminate the collection of content information, FISC judges can insist that the government assess the risk of intercepting content information 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 substantial amount of content information.
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 employed under circumstances, or in a manner, that unreasonably intrudes on individuals’ privacy interests.
In sum, we hold that the request in this ease 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 prоbable cause, simply because of the risk that some content information will be incidentally intercepted as well.
IV
We conclude that Congress intended to minimize the collection of content information by insisting that reasonably available technology be used to segregate dialing information from content information. The government represents.—and we have
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 legitimate 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 certified question in this matter as follоws: the FISC may authorize the collection and decoding of post-cut-through' digits as long as the government is prohibited from making investigative or evidentiary use of any content information 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 information.
Notes
. 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.
. 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,
. 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 substance, purport, or meaning of [a wire, oral, or electronic] communication.”
. The amicus curiae argues that such technology already exists: the government can limit the collection of digits to the first ten dialed digits. To be sure, that approach 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
. Thе 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 analysis 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.
. 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,
The amicus curiae argues that all post-cut-thr.ough 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 meaning of [a wire, oral, or electronic] communication.”
. 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, [redacted] Nonetheless, the amicus argues that the prospect 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 technological 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.
. The problem of pen registers intercepting "content” or "transactional” information was discussed throughout the Joint Hearing on the bill that became the 1994 statute. 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, 103 d Cong. 39-40, 50, 110-11, 114, 116, 158, 161 (1994).
. The term "minimization” has a familiar meaning in the context of interceptions of electronic communications. Section 2518(5) of title 18 directs that electronic surveillance 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.
. The dictum in Zweibon was not joined by a majority 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 collaboration 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.” Halkin v. Helms,
. Consistent with this "primary purpose” requirement, the court affirmed the exclusion of evidence gleaned after the date when the government had "begun to assemble a criminal prosecution.” Truong, F.2dat916.
. 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,
. The government advises us that in the course of its pen register investigations, no such order has ever been granted; in fact, the government has never even sought such an order. See also Record on Appeal, Certification at 2 n.l.
. 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.