The way police seek location information and data from tech companies to investigate crimes is under a microscope at the highest US court, in a closely watched case with broad implications for data privacy and law enforcement.

After about 2 hours of oral arguments in a case involving geofence warrants and Google, however, it was unclear whether the Supreme Court would take any action that could shift interpretations of the Fourth Amendment, which prohibits the government from conducting "unreasonable searches and seizures."

The hearing Monday before the court was for Chatrie v. United States, which centers on a 2019 bank robbery in Richmond, Virginia, where $195,000 was stolen. When the case had gone cold, police obtained a geofence warrant from Google, granting law enforcement access to location data from and around the bank. Using the data, police were able to narrow the list of potential suspects in the area from 19 to three, eventually arresting Okello T. Chatrie, the plaintiff in the case. 

Geofence warrants are often described as "digital dragnets" by critics because they can ensnare innocent bystanders near a crime scene. 

Here's how it works. If there are no clear suspects for a crime, law enforcement can serve a warrant to a tech giant requesting location data. Police draw a circle on a map around a crime scene and specify a time window. The tech company (most frequently Google) searches its database for devices inside that "fence" during that time. The police can then ask the company for the specific account details -- such as email addresses, phone numbers and usernames -- of anyone who appears to be acting suspiciously.

A constitutional issue

The arguments on Monday largely focused on whether geofence warrants themselves are constitutionally problematic and whether location data should be treated the same as other types of data, such as emails or photos stored on the cloud. Much discussion centered on whether the Fourth Amendment applied at all if a person could simply turn off location tracking. 

Adam Unikowsky, lawyer for the plaintiff, argued that because even anonymized location data can be used to identity someone, especially within the confines of a geofence, those who use services like Google Maps may not even be aware that they could be investigated later if they were near a crime at the time it happened.

In this case, Google provided police with a 2-hour window of location data that included, in part, location information for people sitting in a church near the bank who wouldn't be considered suspects or witnesses in the bank robbery.

"Anonymity is somewhat illusory because one's movements within the geofence often functions as a kind of fingerprint," Unikowsky said. "You can figure out who the person is based on a fairly small amount of movements."

The justices grappled with comparisons between location technology and how Google stores it, and with physical-world scenarios involving safety deposit boxes or storage units that police might search with a warrant. 

Some of those concerns form the basis for the argument that geofence warrants could violate the Fourth Amendment. Usually, police must show probable cause that a person committed a crime before they can search their data. A geofence warrant is inherently overbroad, granting law enforcement access to the data of potentially hundreds of innocent people, such as nearby residents or shoppers, to find one suspect.

A spokesperson for the Electronic Privacy Information Center said the Chatrie v. United States case is important because it examines how much latitude the government has to use location services to identity a suspect. Alan Butler, EPIC's executive director and president, said in a statement that geofence searches are "an incredibly invasive investigative technique that threatens the Fourth Amendment rights of hundreds of millions of individuals."  

Butler went on to say: "The court should hold that the Constitution protects our digital data even when it is stored by an app or cloud provider. The court should ensure that the highly sensitive records generated by our phones cannot be obtained without particularized suspicion and close judicial oversight."

What's next for geofence warrants

The Supreme Court has a few options as it considers the case. It could rule that geofence warrants must be more specific about what law enforcement is seeking, rather than casting a wide net and seeing what turns up. Or it could file an opinion that more broadly speaks to what law enforcement can or can't request in warrants seeking data from tech companies, which could have big implications beyond just location data.

It could also maintain the status quo by declining to rule on the case. 

A lawyer for the Department of Justice argued to distinguish location data from more sensitive data, suggesting that it represents "public movements" that others could observe, making it fundamentally different from the "contents of one's personal thoughts." He suggested that a ruling that modifies how warrants are sought for data could have wide-ranging implications for law enforcement, making it more difficult to access evidence such as surveillance footage.

Justice Samuel Alito explicitly told the government that the court shouldn't be writing a "treatise" on the Fourth Amendment in the digital age. He resisted the push for a broad ruling that might inadvertently affect other tools, such as emails, calendars or tower dumps -- a common police tactic in which they identify every phone that connected to a specific cell tower at a specific time. 

Alito also pointed out that because Google has shifted location data from its Sensorvault servers to devices in recent years, the issue of geofence warrants may be moot in some cases. 

Justice Sonia Sotomayor suggested that for such warrants to be constitutional, there must be a "reasonable probability" of finding evidence at each stage of the process and that judges must provide more rigorous supervision over how law enforcement narrows its list of targets. 

"There is value in our setting the parameters of this process, isn't there?" she said.