The new details are the latest leaked by a 29-year-old former NSA contractor who fled to Hong Kong and has been divulging previously secret programs for collecting U.S. phone records and Internet data.
President Barack Obama and other top officials have defended the programs, which again have raised the debate over national security and individual privacy.
Attention turned Thursday to the security clearance background check conducted on Edward Snowden, and a government watchdog testified there may have been problems with it.
USIS, the company that conducted the security clearance investigation of the former NSA systems analyst, is now under investigation itself, Patrick McFarland, the U.S. Office of Personnel Management's inspector general, told a Senate hearing. He declined to say what triggered the inquiry, but when asked if there were concerns about Snowden's background check, McFarland answered: "Yes, we do believe that there may be some problems."
USIS said in a statement that it has never been informed that it is under criminal investigation, and it declined to comment on whether it conducted a background investigation of Snowden.
The new documents revealed by The Guardian concern the scope of two recently disclosed NSA programs — one that gathers U.S. phone records and another that is designed to track the use of U.S.-based Internet servers by foreigners with possible links to terrorism.
The documents were signed by the country's top lawyer, Attorney General Eric Holder. They include point-by-point directions on how an NSA employee must work to determine that a person being targeted has not entered the United States.
If the NSA finds the target has entered the U.S., it will stop gathering phone and Internet data immediately, the documents say. If supervisors determine that information on a U.S. person or a target who entered the U.S. was intentionally targeted, that information is destroyed, according to the documents.
But if a foreign target has conversations with an American or a U.S.-based person whom NSA supervisors determine is related to terrorism, or the communication contains significant intelligence or evidence of crimes, that call or email or text message can be kept indefinitely.
Encrypted communications also can be kept indefinitely, according the documents.
Administration officials had said the U.S. phone records NSA gathered could only be kept for five years. A fact sheet those officials provided to reporters mentioned no exceptions.
The documents outline fairly broad authority when the NSA monitors a foreigner's communications. For instance, if the monitored foreigner has been criminally indicted in the U.S. and is speaking to legal counsel, NSA has to cease monitoring the call. The agency, however, can log the call and mine it later so long as conversation protected by attorney-client privilege is not used in legal proceedings against the foreigner.
The NSA had no comment when asked about the newly revealed documents.
Booz Allen Hamilton, the company where Snowden was working at the time of the disclosures, fired him for violations of the firm's code of ethics and firm policy. The company said he had been a Booz Allen employee for less than three months.
Snowden worked previously at the CIA and probably obtained his security clearance there. But like others who leave the government to join private contractors, he was able to keep his clearance after he left and began working for outside firms.
Of the 4.9 million people with clearance to access "confidential and secret" government information, 1.1 million, or 21 per cent, work for outside contractors, according to a January report from the Office of the Director of National Intelligence. Of the 1.4 million who have the higher "top secret" access, 483,000, or 34 per cent, work for contractors.
Associated Press writers Richard Lardner and Kimberly Dozier contributed.