When Gavin McInnes—founder of the violent, far-right group The Proud Boys—spoke to a Manhattan Republican club last October, the neighborhood response was less than welcoming. Protesters took to the normally sedate Upper East Side block with chants and spray paint. The Proud Boys responded with fists and kicks. Nearly a year later, as the assault and riot charges against four Proud Boys go to trial, prosecutors revealed that they had turned to an alarming new surveillance tool in this case: a reverse search warrant.

The Manhattan District Attorney's Office admitted it demanded Google hand over account information for all devices used in parts of the Upper East Side. They didn’t do this to find the Proud Boys; they did it to find Antifa members.

Reverse search warrants have been used in other parts of the country, but this is the first time one was disclosed in New York. Unlike a traditional warrant, where law enforcement officials request information on a specific phone or individual, reverse warrants allow law enforcement to target an entire neighborhood. Police and prosecutors create a “geofence”—a map area—and demand information on anyone standing in the zone. This flips the logic of search warrants on its head. Rather than telling service providers the name or phone number of a suspect, reverse search warrants start with the location and work backwards.

It’s a big change. Depending on the size and location of the geofence, a reverse search warrant can easily target hundreds or even thousands of bystanders. That scale is what makes reverse search warrants so enticing to law enforcement and so concerning to civil liberties groups. One concern is that the more broadly law enforcement uses surveillance, the higher the risk for “false discovery.” That’s a clinical way to say that the more people you spy on, the more innocent people will wrongly go to jail.

The phenomenon is well-documented in the sciences, where researchers have long known that “high false discovery rates occur when many outcomes of a single intervention are tested.” Essentially, when you look for too many patterns at the same time, you increase the danger that the data will fool you. When police officers request the data for hundreds or even thousands of devices, there’s a higher chance that they’ll wrongly think that one of those bystanders is a suspect.

This isn’t just theoretical. That’s what Jorge Molina discovered in 2018, when Arizona detectives wrongly arrested him for a brutal murder, jailing him for nearly a week before he was exonerated. Officers demanded that Google hand over information on every single laptop, phone, tablet, and smart device in a two-block area. We don’t know how many accounts that includes, but it’s no surprise that while sifting through that many devices that they quickly found a “match.” Only he was innocent.

In response to the Manhattan DA’s reverse search warrant, Google provided information that investigators used—along with images given to a private facial recognition company—to target two people who turned out to be innocent bystanders. Thankfully, unlike in Molina’s case, the two “matches” in Manhattan were never arrested—and the Antifa members have not been identified, even as several Proud Boys have stood trial.

But with the seal broken now in Manhattan, there are likely to be more geofence warrants and more false discoveries. While a judge needs to sign off on a reverse warrant, that formality provides little protection to the public. A traditional warrant application asks for information about the individual being targeted and the reasons they are suspected. With reverse warrants, judges don’t even know how many people’s data will be compromised. They simply don’t have enough information to do their job.

It’s also unclear how judges will evaluate reverse warrants around sensitive sites: political protests, houses of worship or medical facilities, among others. The practice is even more alarming when you consider the ways that ICE and other federal agencies could use a reverse warrant to pursue their deportation campaigns and target American immigrants.

None of this is to say that reverse search warrants are unique, they are just the latest example of how the surveillance capitalism that powers tech firms can become a tool for the government. Maybe some users who happily hand their data to the tech giants will second guess that choice when they realize how quickly their digital sidekicks can morph into a big brother.

Albert Fox Cahn is the executive director of The Surveillance Technology Oversight Project at the Urban Justice Center, a New York-based civil rights and privacy organization. On Twitter @cahnlawny.