On June 22, the Supreme Court of the United States held that the Fourth Amendment to the U.S. Constitution applies to government searches of historical cellular phone location data. The Court's opinion, written by Chief Justice John Roberts, is the latest to address this critical intersection of the Fourth Amendment and mobile devices.

Chief Justice Roberts, writing for a 5-4 majority, reversed a Sixth Circuit decision in the government's favor. The majority held that individuals possess an expectation of privacy in their cell-phone location data, and that the government's seizure of such records implicates the Fourth Amendment. In so finding, the Chief Justice noted that "a person does not surrender all Fourth Amendment protection by venturing into the public sphere," an apparent departure from precedent permitting the government to monitor an individual's movements, under certain circumstances, without any showing of probable cause. Specifically, Chief Justice Roberts found that the access of cell-site records contravenes an expectation that the government not going to "secretly monitor and catalogue every single movement...for a very long period of time."

The majority specifically rejected the government's invitation to apply the third-party doctrine. That principle, stemming from a line of cases decided in the 1970s, permits federal authorities to access bank and other business records maintained by third parties without Fourth Amendment scrutiny. Its rationale—that an individual possesses a reduced expectation of privacy in information voluntarily shared with another—does not, according to the Chief Justice, stand up to scrutiny in the context of cellular location data. Chief Justice Roberts found that the records at-issue contained "a detailed chronicle of a person's physical presence compiled every day, every moment, over years." In his view, such a record "implicates privacy concerns" far beyond the bank records that gave rise to the doctrine.

Furthermore, Chief Justice Roberts noted that cell-site data is not "truly 'shared' as one normally understands the term," and are thus further distinguishable from bank records. Citing Riley v. California, a 2014 decision also concerning the intersection of the Fourth Amendment and cellular phones, the Chief Justice noted that carrying a phone is "indispensable to participation in modern society," and that a cell phone "logs a cell-site record by dint of its operation without any affirmative act on the part of the user." Thus, the majority held, "in no meaningful sense does the user voluntarily 'assume the risk' of turning over a comprehensive dossier of his or her physical movements."

The majority also concluded that the government's access of location data required the issuance of a warrant supported by probable cause. In so doing, the Court held that standard for obtaining an order pursuant to the Stored Communications Act—"reasonable grounds for believing that the records were 'relevant and material to an on-going investigation'"—fell "well short" of the probable cause requirement. He specifically rejected an argument advanced by Justice Alito: that the warrant requirement yields when the government obtains records using compulsory process (i.e. a subpoena, as was done in Carpenter's case). The Chief Justice dismissed this argument as leading to an absurd result, ultimately concluding: "if the choice to proceed by subpoena provided a categorical limitation on Fourth Amendment protection, no type of record would ever be protected by the warrant requirement."

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