While the system is designed to seek foreign intelligence, in some cases it retains the contents of emails between U.S. citizens and filters Internet-based phone calls, Journal reported Wednesday.
The information is gathered from major telecommunications companies at major Internet junctions around the United States, said former intelligence and government officials and people from companies that build or operate the system.
NSA spokesman Vanee Vines said the agency uses "minimization" procedures approved by the U.S. attorney general to protect the privacy of Americans whose personal information is "incidentally collected during NSA's lawful signals intelligence activities."
Another U.S. official said the NSA is "not wallowing willy nilly" through citizens' everyday electronic communications. "We want high-grade ore," the official said.
Documents released by fugitive former NSA contractor Edward Snowden show programs used by the agency have the capability to track virtually anything online covered by a court order.
Much, but not all, of the data is discarded, raising concerns among some lawmakers and civil libertarians existing privacy protections may be inadequate.
U.S. telecommunications companies must turn over information to the NSA that is specified in court orders issued by the secret Foreign Intelligence Surveillance Court. Lawyers at the companies decide what information is considered "responsive" to the court order, an official said.
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