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The federal government purchased access to a database that tracks millions of cell phones and is using the data as part of its ongoing crackdown on undocumented immigrants, according to the Wall Street Journal.

The data is drawn from inconspicuous cell phone apps, like games and weather apps, that ask the user’s permission to access their location. But the data has been used by DHS to “help identify immigrants who were later arrested,” and by CBP to identify cell activity in places such as remote desert areas on the Mexican border, according to the Journal, which said it both reviewed documents and spoke to people “familiar with the matter.”

Notably, ICE was reportedly first given access to the data for usage in anti-human trafficking and drug smuggling efforts, but later began using the data to carry out deportations. The agency wouldn’t confirm or deny that.

“We do not discuss specific law-enforcement tactics or techniques, or discuss the existence or absence of specific law-enforcement-sensitive capabilities,” ICE spokesperson Bryan Cox told the Journal.

The Supreme Court ruled in 2018 that cellular data obtained from wireless carriers requires a warrant under the 4th Amendment. But by purchasing the data from firms such as Venntel like any private company would, the data has been approved for usage by government lawyers, according to the Journal.

Calls to Sen. Ron Johnson of Wisconsin and Rep. Bennie Thompson of Mississippi, the Senate and House chairs of the congressional committees that oversee DHS, were not immediately returned.

At least one privacy advocate said that the news was alarming. “This is a classic situation where creeping commercial surveillance in the private sector is now bleeding directly over into government,” Electronic Privacy Information Center general counsel Alan Butler told the Journal.