The Fourth Amendment in Eight Cases

The Fourth Amendment is 54 words long. It protects "persons, houses, papers, and effects" against "unreasonable searches and seizures" and says warrants need probable cause and must particularly describe what is to be searched. Every argument about plate readers, face scans and phone data comes back to those words. And the words were written for a world of wax seals and horse-drawn carts, so courts have had to build tests on top of them.

You do not need a law degree to follow a surveillance fight. You do need to know which test a court used, what it decided, and what it carefully left alone. Most bad claims about surveillance law, from either side, fail on that last part.

Two tests for a search

The amendment only kicks in when the government does a search or a seizure. If an action is not a search, no warrant is needed. So the first fight in nearly every case is over that word.

The older test is about property. If police physically intrude on your home, your papers or your car to get information, that is a search. For most of American history this was the whole test.

Then came Katz v. United States in 1967. FBI agents taped a listening device to the outside of a public phone booth and recorded Charles Katz placing illegal bets. No trespass into anything he owned. The Court said it did not matter: "the Fourth Amendment protects people, not places." Justice Harlan's concurrence wrote the version courts still quote. A search happens when a person has an actual expectation of privacy and society is prepared to call that expectation reasonable. That is the reasonable expectation of privacy test, and it is slippery on purpose.

The third-party doctrine

A decade later the Court drew a line that powers much of modern surveillance. In United States v. Miller (1976), investigators got a man's bank records from his bank with a subpoena. In Smith v. Maryland (1979), police had the phone company install a pen register to record the numbers dialed from a robbery suspect's home phone. Both times the Court found no search. You gave that information to a business, the reasoning went, so you took the risk the business would hand it over.

That is the third-party doctrine. Think about what it covers today: your email provider, your phone carrier, your grocery loyalty card, your car's connected app, the doorbell company's cloud. If the doctrine applied without limit, almost nothing digital would be protected.

The digital turn

The Court has spent the last 15 years deciding how far to stretch the old rules.

Jones matters because of the concurrences. Justice Alito, writing for four justices, said long-term GPS monitoring would impinge on expectations of privacy even without the physical tracker. Justice Sotomayor agreed. Those five votes pointed toward the idea that collecting many small public facts can add up to something private. Scholars call this aggregation theory. You will see it argued in every plate reader case.

Riley is the case you cite when someone says a phone is just like a wallet. Chief Justice Roberts wrote for a unanimous Court that a modern phone holds "the privacies of life," and the answer to what police must do before searching one is simple: get a warrant.

Carpenter is the big one. The FBI got Timothy Carpenter's cell-site records with a court order under the Stored Communications Act, 18 U.S.C. 2703(d), which only needs "reasonable grounds" to believe the records are relevant. The records covered 127 days and 12,898 location points. The 5 to 4 majority, written by Roberts, held that this was a search and that the 2703(d) standard falls short of probable cause. The third-party doctrine did not apply to this kind of detailed, automatic location trail.

The 2026 geofence decision

A geofence warrant asks a company for every device that was inside a drawn area during a window of time. In 2019, police investigating a bank robbery in Virginia got one from Google for the area around the bank. The warrant worked in steps: anonymized device list first, then more data on some devices, then names for a few. Okello Chatrie was identified and convicted.

Courts split hard. In 2024 the Fifth Circuit, in United States v. Smith, called geofence warrants unconstitutional general warrants. In 2025 the full Fourth Circuit affirmed Chatrie's conviction in a one-sentence order, with separate opinions splitting 7 to 7 on whether a search had even occurred. The Supreme Court granted review on January 16, 2026, heard argument April 27, and decided on June 29, 2026.

The vote was 6 to 3. Justice Kagan wrote the opinion of the Court, joined by Roberts, Sotomayor, Kavanaugh and Jackson; Gorsuch agreed with the result in a separate opinion concurring in the judgment. The holding: police conducted a search when they got Chatrie's Location History from Google, and a person has a reasonable expectation of privacy in that cell phone location data. The Court said no good reason exists to treat Location History differently from the cell-site records in Carpenter. Two hours of data was enough.

Then the part people most often skip. The Court did not decide whether the warrant met the particularity and probable cause requirements at each step. It did not decide the good-faith exception. It vacated the judgment and sent the case back to the Fourth Circuit. Justice Jackson, joined by Sotomayor, would have gone further and found steps two and three invalid. Justice Alito's dissent argued the good-faith exception independently supported the conviction. Justice Barrett wrote that Chatrie had no reasonable expectation of privacy in data about public movements he chose to share with Google.

Other courts to know

Lower courts apply these cases to new tools all the time. In Leaders of a Beautiful Struggle v. Baltimore Police Department (2021), the full Fourth Circuit voted 8 to 7 that Baltimore's aerial surveillance plane program, which photographed much of the city by day, was a search under Carpenter because the data let police retrace people's movements. Plate reader cases have mostly gone the other way so far. Lesson 3 covers a January 2026 federal ruling that a city's network of about 200 plate cameras was not a search.