When a crime has been committed but cops don’t have any suspects, they ask Google and others to canvass the area for them. Officers hand providers a geofence and ask for everyone who wandered into the selected area during a certain timeframe. Once cops have everyone, they start looking for someone.

It seems weird but it’s really not all that different than sending officers out to areas around crime scenes to ask anyone if they’ve seen anything. The difference here is cops are getting info about people’s movements in an area when they’re not suspected of any criminal activity. In addition, the areas covered by warrants often include high-traffic areas like businesses and multi-family housing, increasing the chance cops are going to zero in on the wrong suspect simply because someone lives or works near a crime scene.

That’s exactly what happened in Phoenix, Arizona. Detectives investigating a shooting handed a reverse warrant to Google. This data was used to arrest the wrong person, as Jennifer Valentino-DeVries reports for the New York Times.

The police told the suspect, Jorge Molina, they had data tracking his phone to the site where a man was shot nine months earlier. They had made the discovery after obtaining a search warrant that required Google to provide information on all devices it recorded near the killing, potentially capturing the whereabouts of anyone in the area.

[…]

But after he spent nearly a week in jail, the case against Mr. Molina fell apart as investigators learned new information and released him. Last month, the police arrested another man: his mother’s ex-boyfriend, who had sometimes used Mr. Molina’s car.

Police like the new tool, but acknowledge it has its limits. First, there’s a lot of junk data to sort through, and not all investigators have the training necessary to make sense of these quasi-tower dumps. But that hasn’t stopped them from using this option with increasing frequency. Google — which holds up to ten years of location data in a massive database referred to by employees as the “Sensorvault” — is now receiving up to 180 requests a week for mass location data.

Prosecutors seem cautious about the tool as well, which is a good sign. But considering what happened to Jorge Molina, this statement seems a bit too optimistic.

“It doesn’t pop out the answer like a ticker tape, saying this guy’s guilty,” said Gary Ernsdorff, a senior prosecutor in Washington State who has worked on several cases involving these warrants. Potential suspects must still be fully investigated, he added. “We’re not going to charge anybody just because Google said they were there.”

Well, maybe Ernsdorff won’t. But it appears others will. The more law enforcement shoots first and asks questions later, the more often it’s going to end up putting the wrong person behind bars. Like any other bulk collection, the potential to reach the wrong conclusion while sorting through haystacks is omnipresent. If Google’s responding to tens of thousands of requests a year for bulk location data, Jorge Molina’s story isn’t going to remain an outlier for long. Certainly the tool has proven useful in other cases, but the sheer number of requests suggests law enforcement isn’t using this tool cautiously. That’s bad news for everyone carrying a cellphone.