Documents released by the US government show it views an executive order issued in 1981 as the basis of most of the National Security Agency's surveillance activities, the American Civil Liberties Union said on Monday.
The NSA relied on Executive Order 12333 more than it did on two other laws that have been the focus of public debate following the leaks exposing US surveillance programs by former agency contractor Edward Snowden, according to the papers released by the ACLU.
The ACLU obtained the documents after filing a lawsuit last year seeking information in connection with the order, which it said the NSA was using to collect vast amounts of data worldwide, "inevitably" including communications of US citizens.
The order, signed in 1981 by President Ronald Reagan, was intended to give the government broad authority over surveillance of international targets.
One of the documents obtained was a 2007 NSA manual citing the executive order as "the primary source of NSA's foreign intelligence-gathering authority."
A legal fact sheet on the memo produced in June 2013, two weeks after Snowden's disclosures, said the NSA relied on the executive order for the "majority" of its activities involving intelligence gathered through signals interception.
Alex Abdo, an ACLU staff attorney, said in a blog post published on Monday that the documents "confirm that the order, although not the focus of the public debate, actually governs most of the NSA's spying."
"Congress's reform efforts have not addressed the executive order, and the bulk of the government's disclosures in response to the Snowden revelations have conspicuously ignored the NSA's extensive mandate under EO 12333," Abdo wrote.
Neither the NSA nor US Department of Justice, which is defending the lawsuit, responded to requests for comment Monday.
The ACLU's lawsuit, filed in December 2013 in New York, cited news reports indicating that, under the order, the NSA is collecting data on cell phone locations and email contact lists, as well as information from Google and Yahoo user accounts.
In his blog post, Abdo said the documents made clear the government is collecting information that goes beyond just terrorist threats.
The documents, he wrote, could also add credence to concerns the government could use its "broad surveillance power to conduct economic espionage or to spy on Americans it hoped to convert into confidential informants."
An internal US Defence Department presentation released in response to the lawsuit said it could, under the order, collect information on US citizens or organizations in a number of scenarios.
Those include if a commercial organisation is believed to have ties to foreign organisations or people and "potential sources of assistance to intelligence activities," the document said.