The US supreme court has ruled that law enforcement’s use of sprawling warrants that sweep up smartphone location data requires privacy protections under the fourth amendment, in a boost to critics who view their use as an unconstitutional dragnet.

Justice Elena Kagan wrote the majority opinion, which held that the sensitive data scooped up by “geofence warrants” counts as a fourth amendment search, and offers individuals a “reasonable expectation of privacy”, even if they may be in a public area.

“An individual has a reasonable expectation of privacy in records about his cell phone’s location, and police intrude on that constitutionally protected interest when they demand the information – even though for only a limited time, and from a third-party tech company,” Kagan wrote.

The judges ruled 6-3 in Chatrie v US, against the government, in a case that has been widely viewed as a test of how privacy rights translate into a new digital era.

The use of geofence warrants is widespread, and gives law enforcement agencies the power to compel tech companies to hand over sensitive cell phone data from people at or near crime scenes. The warrants allow police and the FBI to collect this information from individuals within the radius of a virtual “fence” during a particular timeframe. But they aren’t restricted to requesting data for precise targets.

The Chatrie case focuses on local police’s pursuit of an armed bank robber in Richmond, Virginia. He fled with $195,000. Law enforcement tracked Okello Chatrie down through their use of geofence warrants. Chatrie had opted in to an optional Google “location history” feature that documented his location every few minutes. He was eventually sentenced to 12 years in prison, after pleading guilty.

Chatrie’s lawyers argued that this search was overly broad and violated his fourth amendment rights, which protects individuals from “unreasonable search and seizure”. Lawyers said that police’s use of geofence warrants amounted to an official “search” under the fourth amendment, and didn’t meet the constitution’s requirements for one.

The government had argued that accessing only a short amount of cellphone location information means this tactic does not count as a fourth amendment search and accordingly, should not be afforded the same privacy protections. But the judges in the majority disagreed.

The judges in the majority opinion also wrote that the government’s characterization of generating location history as a voluntary choice is “meritless”.

They suggested that people aren’t choosing to share private information with third parties and the government “just by doing the ordinary thing cellphone users do.” “The point of carrying smartphones is to use what is on them”, including the apps and services they provide – many of which use location data to customize a user’s experience, they said.

“That argument ignores how and why Google users turn on location history: Google repeatedly prompts users to turn on the service, often warning that devices will not “work correctly” otherwise, while not disclosing in that prompt how frequently users’ location information would be recorded, how precise it would be, or how it might be given to the government,” they wrote.

Justice Sonia Sotomayor wrote that “even short-term monitoring” of a person’s physical movements can provide “a wealth of detail about [his] familial, political, professional, religious, and sexual associations”. She highlighted examples of a person’s trips to “the psychiatrist, the plastic surgeon, the abortion clinic, the Aids treatment center, the strip club, the criminal defense attorney, [or] the by-the hour motel”.

Privacy advocates share her concerns that geofence warrants can be overly broad in the area they target, as well as the length of time they cover. “If the government doesn’t need to … link something to a crime, it could monitor a protest or an abortion clinic or a gun range or a church or an AA meeting or a doctor’s office,” says Matthew Tokson, a law professor at the University of Utah.

While the majority opinion noted that police conducted a fourth amendment search by accessing Chatrie’s location history data, they noted that the court of appeals will weigh in on whether the “search was reasonable, meaning that each of its steps was properly described with particularity and found to be supported by probable cause”.

Law enforcement has said they need geofence warrants to find suspects and witnesses – after reaching dead ends. The US government, for its part, has argued that people can’t have a “reasonable expectation of privacy” when they are in public and have allowed a third party company, such as Google, to collect and analyze phone location data.

The government noted in its legal filings that “only about one-third of active Google account holders actually opted into the location history service”; Chatrie’s lawyers noted in court documents that this amounted to more than 500 million Google users. Even Google has acknowledged in legal filings for the case that geofence searches “often run a high risk of sweeping in innocent users–sometimes thousands of them.” The tech company said that it’s common for these inquiries to cover private homes, apartment buildings, government buildings, hotels, places of worship, busy roads, and other locations that law enforcement hasn’t identified probable cause to search.

The ruling marks the first time the US supreme court is considering the scope of the fourth amendment since a landmark 2018 privacy ruling. At the time, judges decided in a 5-4 decision that the government generally needs a warrant to track a person’s cellphone location history.

Paul Ohm, a law professor at Georgetown University, said “today is a very good day for constitutional privacy.” He added: “The court reaffirmed that the police need a search warrant to turn private services like Google’s location tracking into a state surveillance tool.”