After Carpenter: The Fourth Amendment in the Age of Involuntary Surveillance

When Chief Justice Roberts announced the majority opinion in Carpenter v. United States, 138 S. Ct. 2206 (2018), he did something the Supreme Court had not done in decades: he declined to extend the third-party doctrine. Cell-site location information — seven days of it, in Carpenter's case — was different. The government needed a warrant. The third-party doctrine, which had permitted warrantless acquisition of bank records (United States v. Miller, 425 U.S. 435 (1976)) and dialed phone numbers (Smith v. Maryland, 442 U.S. 735 (1979)), would not carry this weight.

The decision was 5-4. The reasoning was narrow. And the questions it left open have occupied the lower courts ever since.

First: how long is too long? The Carpenter Court emphasized the "depth, breadth, and comprehensive reach" of seven days of location data. But it declined to say whether a shorter period — a day, an hour, a single ping — would also require a warrant. The government has argued, with some success in the lower courts, that short-duration CSLI collection falls outside Carpenter's warrant requirement. The defense bar argues that the logic of Carpenter — that location data reveals "the privacies of life" — applies regardless of duration.

Second: what about tower dumps? A tower dump reveals every device that connected to a particular cell tower during a particular time window. The government routinely obtains tower dumps without a warrant, arguing that they are analogous to pen registers — capturing connection data, not location data. But a tower dump, by definition, reveals that every device in the dump was at or near the tower at the relevant time. That is location data. The Seventh Circuit has held that Carpenter applies to tower dumps; the Fifth Circuit has held the opposite. The split is ripe for Supreme Court review.

Third: what about real-time tracking? Carpenter was about historical CSLI — records of where the defendant had been, not where he was going. The government continues to obtain prospective location data — real-time CSLI, GPS pings from vehicles, and geofence warrants — under a patchwork of statutory authorities and court orders. Whether the Carpenter warrant requirement extends to prospective surveillance is an open question in most circuits.

For the federal practitioner, the takeaway is clear: every digital evidence case is now a Fourth Amendment case. The motion to suppress that was routine in the pre-Carpenter era — filed and denied as a matter of course — now has real teeth. The government's evidence, more often than not, includes location data. And the law governing that data is unsettled in ways that benefit the defense.