To collect the content of emails, a warrant was required from the most secret bench in the U.S., the Foreign Intelligence Surveillance Court, which convenes in Washington behind closed doors in an eavesdrop-proof chamber. The FISA court every 90 days renewed authority to collect the bulk email metadata – which was gathered primarily for use in counterterrorism – between late 2001 and 2011, when the program was shut down, the Guardian reported.
Most of the communications collected by the NSA were among people outside the U.S. but eventually messages originating inside the U.S. also were gathered, as detailed by the U.K.-based newspaper.
"The revelations about our government's spying raise new and troubling questions about the extent to which the government is monitoring Americans' private lives, including whom we email or chat with and what websites we visit," said Alex Abdo, staff attorney with the American Civil Liberties Union's National Security Project.
The program was terminated in 2011, not because of privacy concerns, but because it "didn't have the operational impact [the NSA] needed," according to NSA chief Gen. Keith Alexander.
"That was a choice that came from us, from NSA," he said today at the Armed Forces Communications and Electronics Association's International Cyber Symposium in Baltimore.
Alexander insisted that NSA did not keep what it had collected before the program ended and that "all that data was purged at that time."
The event at which Alexander spoke was sponsored by communications behemoth Verizon and defense contractor Booz Allen Hamilton. Snowden was a Booz contractor at the NSA and swiped a secret FISA court order directed at Verizon Business Services regarding agency collection of phone metadata from callers both inside and outside the U.S., which was the first leak published by the Guardian this month.
U.S. diplomats, meanwhile, continue to press authorities in Russia – which has no extradition agreement with the U.S., though the two countries do exchange captured fugitives – to send Snowden home to face charges under the 1917 Espionage Act.
"We'd like to see him come home," State Department spokesman Patrick Ventrell said in Washington.
And while some supporters, like Pentagon Papers leaker Daniel Ellsberg, call Snowden a hero for exposing the secret surveillance programs, others, like Secretary of State John Kerry, have said he's putting American lives at risk.
Al Qaeda and its affiliates have been reading the leaked documents and advising violent jihadis to take actions to avoid electronic surveillance by the NSA, a senior U.S. intelligence official told ABC News.
ABC News' Kirit Radia, Luis Martinez, Akiko Fujita and Natalia Osipova contributed to this report.