According to new Washington Post report, the National Security Agency (NSA) tracks the locations of nearly 5 billion cellphones every day overseas, including those belonging to Americans abroad.

The NSA inadvertently gathers the location records of “tens of millions of Americans who travel abroad” annually, along with the billions of other records it collects by tapping into worldwide mobile network cables, the newspaper said in a report on its website.

Such data means the NSA can track the movements of almost any cellphone around the world, and map the relationships of the cellphone user. The Post said a powerful analytic computer program called CO-TRAVELER crunches the data of billions of unsuspecting people, building patterns of relationships between them by where their phones go. That can reveal a previously unknown terrorist suspect, in guilt by cellphone-location association, for instance.

Former NSA systems analyst Edward Snowden

As the NSA doesn’t know which part of the data it might need, the agency keeps up to 27 terabytes, or more than double the text content of the Library of Congress’ print collection, the Post said. A 2012 internal NSA document said the volumes of data from the location program were “outpacing our ability to ingest, process and store” it, the newspaper said.

The program is detailed in documents given to the newspaper by former NSA systems analyst Edward Snowden. The Post also quotes unidentified NSA officials, saying they spoke with the permission of their agency.

Shawn Turner, a spokesman for the Office of the Director of National Intelligence, declined to comment on the report.

The DNI’s general counsel, Robert Litt, has said that NSA does not intentionally gather bulk location data on US cellphones inside the US — but NSA Director Keith Alexander testified before Congress his agency ran tests in 2010 and 2011 on “samples” of US cell-site data to see if it was technically possible to plug such data into NSA analysis systems.

Keith Alexander said that the information was never used for intelligence purposes and that the testing was reported to congressional intelligence committees. He said it was determined to be of little “operational value,” so the NSA did not ask for permission to gather such data.

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