Documents leaked by NSA whistleblower Edward Snowden have revealed a special and "highly collaborative" relationship between the spy agency and telecom giant AT&T, which provided "massive amounts of data" to the government over a decade.

The documents show that between 2003 and 2013, AT&T's "extreme willingness to help" allowed the NSA access to billions of emails that circulated through multiple internet servers across America, including communications that flowed in and out of the United Nations headquarters in New York under various legal channels, and secret court orders, a joint _New York Times _and _ProPublica _investigation found.

In that time, AT&T — codenamed Fairview in the documents — reportedly installed surveillance apparatuses at 17 or more of its servers located in the US. The program was almost twice as large and expensive as the agency's second biggest program with AT&T's rival Verizon, which was codenamed Stormbrew in the files.

Neither AT&T nor Verizon were specifically named in the documents, but reporters at the _Times _and _ProPublica _revealed in detail the methodology that they used to ascertain the identities of the two companies. Several former intelligence officials also backed up their findings.

At the onset of the Fairview program, AT&T handed over some 400 billion records containing metadata from emails — which did not include the contents — and also forwarded "more than one million emails a day to the keyword selection system." By 2011, the company was providing the spy agency with records on 1.1 billion domestic cellphone calls a day, one internal agency document revealed. Another presentation document showed that in that same year, the agency spent $188.9 million on the Fairview program — almost double what it spent on the Stormbrew operation.

The files further showed that AT&T's unique relationship with the NSA allowed the agency access to web traffic from other companies through its so-called "peering" network, a voluntary connection of separate traffic networks.

Agency documents indicate that the telecom partners sifted through the data before they sent it to the NSA. By 2013, that included some 60 million email exchanges between foreigners abroad that passed through American servers belonging to AT&T. Verizon reportedly only began performing similar operations for the NSA in March 2013.

Under the Foreign Intelligence Surveillance Act (FISA), the NSA must get a warrant before targeting anyone on American soil. In communications between an American and a foreigner abroad, the NSA can target the foreigner without first obtaining a court order, while foreign-to-foreign communications are free for bulk collection by the agency.

Snowden, a former NSA contractor who now lives in Moscow, first leaked thousands of pages of classified documents to journalists in 2013. The trove detailed the extent of government monitoring of communications in the years after 9/11. The Snowden documents have led to several policy changes, including the instigation of the Freedom Actin June, which significantly rolled back the NSA's bulk data collection program.

The first of the cache of documents leaked by Snowden appeared in the Guardian. Since then, the 32-year-old has been releasing documents piecemeal to trusted journalists. The Department of Justice has labeled Snowden a spy, charged him with espionage, and insisted he could only return to the US by pleading guilty to felony charges.