Carpenter v. United States, Explained

Every time a phone connects to a cell tower to send or receive a call or a data packet, the carrier logs which tower handled it. Stacked together over weeks or months, those logs sketch out where a person has been, day after day. Before 2018, the government's standard view was that this data belonged to the phone company, not the customer, and so the Fourth Amendment had nothing to say about how the government got it. Carpenter v. United States rejected that view, though only for this specific kind of data.

That distinction matters because most people never think about cell-site records at all. Nobody chooses to generate them the way they choose to post something online; a phone logs a connection automatically, in the background, as a side effect of working. Carpenter is the case that decided whether that automatic, unavoidable byproduct of carrying a phone should get the same weak protection as records someone deliberately hands over to a bank or a business, or something closer to the protection given to a person's own home and papers.

What the government did in this case

Investigators believed Timothy Carpenter was involved in a string of robberies. Instead of getting a warrant, they used the Stored Communications Act, 18 U.S.C. § 2703(d), which lets the government compel a phone company to disclose records on a showing of "reasonable grounds," a lower bar than probable cause. Carriers turned over 12,898 location points for Carpenter's phone spanning 127 days, an average of about 101 data points a day. That volume of detail was central to how the Court saw the case: this was not a single data point but a near-continuous, retrospective map of where Carpenter had slept, worshipped, and spent his time.

Why the Court drew the line here

Decided June 22, 2018, by a 5-4 vote in an opinion by Chief Justice Roberts, Carpenter held that acquiring seven or more days of historical cell-site location information is a search, and generally requires a warrant. The government argued that the third-party doctrine, the rule from cases like Smith v. Maryland and United States v. Miller that people generally lose Fourth Amendment protection in records they voluntarily share with a business, should cover this too, since carriers are the ones who generate and hold the location logs. The Court declined to extend that doctrine to cell-site data, reasoning that it provides a "detailed, encyclopedic" and "effortlessly compiled" chronicle of a person's physical movements that reaches into deeply private territory the third-party doctrine was never designed to expose. The § 2703(d) order the government used, built for a different, lower-stakes kind of record request, was not enough for this.

What the Court deliberately left alone

Carpenter is a narrow, fact-bound decision, and the majority said so directly. It did not disturb the third-party doctrine generally, only carved out this one exception. It expressly limited its holding to historical cell-site records of this kind and said it was not addressing real-time location tracking, tower dumps covering many phones at once, or other novel forms of digital surveillance. Exactly how many days of data are enough to trigger the rule, and whether the same logic extends to other kinds of third-party digital records, were left unsettled and have been worked out gradually since, including through the geofence-warrant litigation covered elsewhere on this site. As of 2026-07-12, the Supreme Court is reported to have applied similar reasoning to a different category of location data, Google's more granular Location History, in a 2026 decision; see the companion article on geofence and keyword warrants for that later, still-developing chapter.