On November 29, many interested audience members packed into the Supreme Court to witness oral argument on the issue of whether the Fourth Amendment demands that the government obtain a warrant in order to acquire long-term, cell-site location information (CSLI) from wireless service providers, in what could be one of the most influential privacy decisions of this generation: Carpenter v. United States.

In the wake of a string of armed robberies at electronic retail stores in the Detroit area in 2011, the Federal Bureau of Investigation (FBI) obtained orders pursuant to the Stored Communications Act (SCA) requesting “transactional records” from wireless service providers, including CSLI pertaining to the call origination and termination of the arrested suspects’ phone numbers. Under the SCA, the government may require disclosure of such records upon a finding of “specific and articulable facts” that “there are reasonable grounds to believe that the contents of a wire or electronic communication, or the records or other information sought, are relevant and material to an ongoing criminal investigation.” 18 U.S.C. 2703(d). Defendant Timothy Carpenter moved to suppress the government’s cell-site evidence before trial, claiming the government obtaining CSLI from his wireless service provider constituted a warrantless search in violation of the Fourth Amendment. The district court denied the motion, the Sixth Circuit affirmed, and the Supreme Court granted certiorari last June.

Carpenter argued that acquisition of longer-term CSLI constitutes a search based on both a reasonable expectation of privacy theory and a property-based theory, saying that such acquisition interferes with the security of an individual’s private papers (an argument that appeared to intrigue Justice Gorsuch), and that the search is unreasonable. On the other hand, the government contended that its acquisition of CSLI was not a search because not only do cell-phone users not have a reasonable expectation of privacy—much less a property-based interest in business records of their wireless service provider—but even if it was a search, obtaining the CSLI pursuant to the compulsory process designed by Congress in the SCA is also reasonable under the Fourth Amendment.

A highlight of the oral argument was discussion relating to Carpenter’s suggested rule of requiring a warrant to acquire more than 24 hours’ worth of CSLI. In particular, Justices Ginsburg and Sotomayor, along with Chief Justice Roberts, questioned the durational line-drawing, and Justice Kennedy pointed out the corroborative benefit of CSLI that the rule would diminish. Justice Sotomayor underscored a distinction between, on the one hand, the incident-related search (requesting records related to an individual crime) at issue in the current case, and on the other, the “dragnet” searches of intimate details unrelated to a legitimate law enforcement purpose with which she was concerned in her United States v. Jones concurrence, 565 U.S. 400, 417 (2012) (Sotomayor, J., concurring). Further in-depth analysis of the oral argument can be found here.

Most analyses of the case, including that on Daniel Solove’s Privacy + Security Blog, have focused on the relevance and scope of the “third party doctrine.” The government emphasized in its brief, quoting Smith v. Maryland, 442 U.S. 735, 743-744 (1979) (records of phone numbers dialed), that the Court “consistently has held that a person has no legitimate expectation of privacy in information he voluntarily turns over to third parties,” and cited United States v. Miller, 425 U.S. 435 (1976) (bank records) for the proposition that the government’s acquisition of records containing such information from third parties that create them is not a search. Carpenter argued that the current case can be distinguished from the “pre-digital” third-party doctrine decisions because CSLI is far more sensitive and not voluntarily conveyed (a point with which Justice Alito said during the arguments that he agreed to disagree), but does not believe the Court must “reconsider the premise” of the entire doctrine or its application to every context—only its application to long-term CSLI (an exception to which Justices Sotomayor and Breyer seemed most receptive).

The decision has the potential to have far-reaching consequences for many issues, from the Fourth Amendment justifications underpinning what Orin Kerr refers to as the “eyewitness rule” and NSA metadata surveillance, to the First Amendment freedoms of journalists and any citizen with a cell phone.