A new report reveals the National Security Agency is tracking billions of cellphones’ location data around the world, confirming the fears of privacy advocates that such information could be used to map relationships among people based on their movements.

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The Washington Post reported Wednesday that top-secret documents leaked by admitted whistleblower Edward Snowden show the NSA collects 5 billion records each day on cellphones worldwide. Although the agency is only legally allowed to target international cellphones without a warrant, it “incidentally” is getting its hands on domestic data as well. Data collected also includes that of Americans who travel abroad, the Post reported of an NSA official speaking on the condition of anonymity.

The bulk collection of this data is sifted through the NSA’s analytic tools, known as CO-TRAVELER, which the Post described as looking for unknown relationships that might be had with targets. According to the Post it, tracks “people whose movements intersect.”

“One of the key components of location data, and why it’s so sensitive, is that the laws of physics don’t let you keep it private,” Chris Soghoian, principal technologist with the American Civil Liberties Union, told the Washington Post. “The only way to hide your location is to disconnect from our modern communication system and live in a cave.”

It is unknown how many Americans’ cellphone location data is tracked incidentally. That said, the Washington Post pointed out that the Snowden-leaked documents show dealing with U.S. location data seems somewhat routine in training material.

The Post also pointed out that the NSA seems to have accounted for those who take extra precautions to protect their location data through encryption or using disposable phones. These actions, the Post reported, serve even more to peg a person for extra attention from the agency.

Read more details about this new revelation in the Washington Post’s full story.