Under mounting pressure from lawmakers, Deputy Attorney General James Cole said the order spells out how the government can use call data obtained from telecom giants such as Verizon.
Cole told senators that the order "provides that the government can search the data only if it has reasonable articulable suspicion that the telephone number being searched is associated with certain terrorist organizations."
Administration officials confirm that an order to compile phone metadata was issued to a subsidiary of Verizon Communications in April, The Washington Post reported.
The order was among three secret documents declassified by the office of Director of National Intelligence James Clapper, which said Clapper "determined that the release of these documents is in the public interest."
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The move to confront growing opposition to the secret programs came as the administration faced new disclosures from Edward Snowden, the former intelligence contractor at the center of the controversy.
The latest documents obtained by him and published by British daily the Guardian revealed a secret surveillance system known as XKeyscore that allows US intelligence to monitor "nearly everything a typical user does on the Internet."
Snowden, who is now stranded in the transit area of a Moscow airport, fled the United States after downloading NSA files that have formed the basis of one bombshell leak after another.
Since he disclosed the vast scale of US electronic surveillance operations in June, public sentiment in the US has shifted against them.