Carpenter v. United States

Investigating a string of robberies, the government obtained months of Carpenter's historical cell-site location information from his wireless carriers under a Stored Communications Act order — a showing well short of probable cause. Under Smith and Miller, records voluntarily conveyed to a third party carry no Fourth Amendment protection. The question was whether that third-party doctrine covers a time-stamped log of everywhere a phone, and so its owner, has been.

The third-party doctrine does not extend mechanically to every class of record a business holds. Given the unique nature of cell-site records — their depth, breadth, comprehensive reach, and the inescapable and automatic character of their collection — the fact that a carrier holds them does not by itself overcome the user's claim to protection. Acquiring such records is a search, and the government must generally obtain a warrant supported by probable cause before compelling their production.

The Court stressed what made CSLI different from the bank records and dialed numbers of the older cases: carrying a phone is indispensable to modern life, the logging happens without any affirmative act by the user, and the resulting record is retrospective, effectively giving the government a time machine over years of movements. The decision was expressly narrow — it did not disturb conventional surveillance, business records generally, or techniques involving foreign affairs or national security — but within its lane it rejected the mechanical application of precedent to a technology those precedents never imagined.

Reversed and remanded: the Stored Communications Act order was not enough, and a warrant is generally required for historical CSLI of this scope. Carpenter is now the pivot for every argument that a new category of aggregated digital records — location, telemetry, health, or platform data — deserves protection the third-party doctrine would once have denied it.