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 print collection of the Library of Congress, 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 U.S. cellphones inside the U.S. but NSA Director Keith Alexander testified before Congress his agency ran tests in 2010 and 2011 on "samples" of U.S. cell-site data to see if it was technically possible to plug such data into NSA analysis systems. 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.
Sen. Ron Wyden, a Democrat and a member of the Senate Intelligence Committee, said at the time that Alexander could have explained more. "The intelligence leadership has decided to leave most of the real story secret," Wyden said, though he would not elaborate on the extent of the program. Wyden and two other Democratic lawmakers have introduced an amendment to the 2014 defense spending bill that would require intelligence agencies to say whether the NSA "or any other element of the intelligence community has ever collected the cell-site location information of a large number of United States persons with no known connection to suspicious activity, or made plans to collect such information."
Alexander and other NSA officials have explained that when U.S. data are gathered "incidentally" overseas, it is "minimized," meaning that when an NSA analysts realize they are dealing with a U.S. phone number, they limit what can be done with it and how long that data can be kept.
Rights activists say those measures fall short of protecting U.S. privacy.
"The scale of foreign surveillance has become so vast [that] the amount of information about Americans 'incidentally' captured may itself be approaching mass surveillance levels,'" said Elizabeth Goitein of the Brennan Center for Justice's Liberty and National Security Program.
"The government should be targeting its surveillance at those suspected of wrongdoing, not assembling massive associational databases that by their very nature record the movements of a huge number of innocent people," said Catherine Crump, a staff attorney for the American Civil Liberties Union.