The Department of Homeland Security has been purchasing cellphone location data and using it to track activity near the US-Mexico border, according to The Wall Street Journal. The data has reportedly led to arrests after law enforcement saw where people were crossing the border and traced the data back to specific people.

The location data comes from a commercial database composed of information compiled on users by marketing companies. Advertisers and app developers are often able to gather far more information than a user might realize, such as once they’ve granted an app permission to use their location for a more legitimate use, like checking the weather.

Marketing data can create an extremely revealing picture

All of this data collection can lead to an incredibly revealing portrait of an individual’s behavior being created, even though they might have little idea that they, theoretically, consented to the information being shared. It also means that the government can obtain very revealing data on a broad swath of people without going through the courts or relying on questionable legal precedents. It can just buy the information outright, like anyone else can.

The Department of Homeland Security confirmed to the Journal that it had purchased the data. Immigration and Customs Enforcement and Customs and Border Protection are reported to have used the data but did not specifically acknowledge doing so.

The Journal says the data came from Venntel, a company that calls itself a “pioneer in mobile location information” that “supports our national interests through technological innovation.” Venntel indicated that the Department of Homeland Security was a customer but otherwise declined to comment to the Journal.

The New York Times recently highlighted just how revealing this information can be. With access to a database of cellphone location data, reporters were able to track even high-profile individuals in great detail. “We followed military officials with security clearances as they drove home at night. We tracked law enforcement officers as they took their kids to school,” wrote Stuart A. Thompson and Charlie Warzel.