The US National Security Agency can keep copies of intercepted communications from or about US citizens if the material contains significant intelligence or evidence of crimes, according to top-secret documents.
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 programmes for collecting US phone records and internet data.
President Barack Obama and other senior officials have defended the programme, which again have raised the debate over national security and individual privacy.
Attention turned to the security clearance background check conducted on Edward Snowden, and a government watchdog testified there might have been problems with it.
USIS, the company that conducted the security clearance investigation of the former NSA analyst, is now under investigation itself, Patrick McFarland, the US 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 Mr Snowden's background check, Mr 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 programmes - one that gathers US phone records and another that is designed to track the use of US-based internet servers by foreigners with possible links to terrorism. The documents were signed by the country's leading 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 US, it will stop gathering phone and internet data immediately, the documents say. If supervisors determine that information on a US person or a target who entered the US was intentionally targeted, that information is destroyed, according to the documents. But if a foreign target has conversations with an American or a US-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 to the documents. Administration officials had said the US 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 US 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.