Illustration by Alex Castro / The Verge

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.

All of this data…

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