Privacy and digital rights advocates are shining a light on a little-known tool used by law enforcement that allows investigators to track cellphones by analyzing phone location data obtained from millions of devices.

The software, called Fog Reveal, has given agencies from across the country access to billions of phone records from over 250 million mobile devices, according to public records and internal emails obtained by AP News.

Police have utilized the technology, created by Virginia company Fog Data Science, LLC, to analyze personal location data and obtain “patterns-of-life” information without having to first obtain search warrants.

Although it’s undoubtedly a convenient tool for law enforcement, not everyone is embracing the controversial technology.

Digital privacy advocates argue that the software allows for unprecedented police surveillance and violates the Fourth Amendment protections against unreasonable search and seizure, while defense attorneys complain that the tool is rarely mentioned in court records and prevents them from adequately defending their clients.

Fog Reveal first came on the scene in 2018. Since then, it’s been successfully utilized in several homicide investigations and, more recently, has been used to track participants in the January 6 Capitol riot.

“It’s sort of a mass surveillance program on a budget,” Bennett Cyphers, a special adviser at the digital rights advocacy group Electronic Frontier Foundation (EFF), explained.

Fog Data Science, LLC was founded by two former high-ranking Department of Homeland Security officials under the Bush administration and is marketed heavily toward law enforcement.

The software relies on advertising identification numbers purchased from popular apps like Waze, Starbucks and countless others that use targeted advertisements based on a person’s movements and interests, police emails say.

With that data, police can obtain location data from specific devices and trace them to home addresses and workplaces.