The Obama administration insists the NSA keeps only the content of messages and communications of people it is intentionally targeting.
The metadata stored by the NSA provides a record of almost everything a person does online, including tracking browsing history, email activity and some account passwords, the top-secret document leaked to British newspaper The Guardian by former NSA contractor Edward Snowden indicates.
The leaked document is an introductory guide to digital network intelligence for NSA field agents, The Guardian said.
Any computer metadata picked up by NSA collection systems is routed to the agency's metadata repository, code-named Marina, the guide explains.
Phone metadata is sent to a separate system called Nucleon.
"The Marina metadata application tracks a user's browser experience, gathers contact information/content and develops summaries of target," the guide says in Tuesday's Guardian report.
"Of the more distinguishing features, Marina has the ability to look back on the last 365 days' worth of DNI metadata seen by the SIGINT collection system, regardless whether or not it was tasked for collection," the guide says, referring in the capitalized initials to the Office of the Director of National Intelligence and to signals intelligence.
Looking back on a person's 365-day history can let the spy agency find information about someone who later becomes a target, The Guardian said.
But having this ability depends on keeping personal information of millions of Internet users who never will be of interest to the U.S. intelligence community, the newspaper said.
The Guardian asked the NSA to explain the 365-day storage of untargeted people's data and to estimate how much U.S. citizens' metadata was kept in its repositories.
The NSA didn't address the questions in its response, focusing instead on its foreign intelligence activities.
"NSA is a foreign intelligence agency," the statement said. "NSA's foreign intelligence activities are conducted pursuant to procedures approved by the U.S. attorney general and the secretary of defense, and, where applicable, the Foreign Intelligence Surveillance Court, to protect the privacy interests of Americans.
"These interests must be addressed in the collection, retention and dissemination of any information," the spy agency said. "Moreover, all queries of lawfully collected data must be conducted for a foreign intelligence purpose.
"We know there is a false perception out there that NSA listens to the phone calls and reads the email of everyday Americans, aiming to unlawfully monitor or profile US citizens. It's just not the case.
"NSA's activities are directed against foreign intelligence targets in response to requirements from U.S. leaders in order to protect the nation and its interests from threats such as terrorism and the proliferation of weapons of mass destruction."