The documents describe how the NSA must first build a case in order to target a foreigner for phone or Internet surveillance.

The top secret documents were published by The Guardian in the latest leak of classified US materials.

WASHINGTON — The 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 exemptions in NSA’s top secret rules published Thursday.

They also describe how the agency is to make sure the person is outside the United States — and not an American.

But if the target is communicating with an American, the record of contact can be kept indefinitely. Administration officials have said material the NSA inadvertently gathers on Americans is destroyed.


The NSA did not immediately respond to requests for comment.

Intelligence officials have said the surveillance of intercepted communications has helped thwart at least 50 plots in 20 countries worldwide, including 10 to 12 plots against the United States.

But the numbers have been questioned by many in Congress, as lawmakers grapple with how to assure Americans that their privacy rights are protected amid the security risks they face.

Obama administration officials are considering whether they should modify the NSA’s routine collection of data on most telephone calls made in the United States, the most controversial aspect of the programs.

One proposal would require the phone companies to store the call data for five years and make it available to government investigators. FBI director, Robert S. Mueller III, said Wednesday that such a change would slow investigators as they seek to stop terrorist attacks.

The Privacy and Civil Liberties Oversight Board, a federal panel that is reviewing the NSA programs, said it would sponsor a public discussion next month on the privacy issues raised by the surveillance programs.


In a separate development Thursday, a government watchdog said the government contractor that performed a background investigation of the man who says he disclosed two NSA surveillance programs is himself under investigation.

Patrick McFarland, the inspector general at the Office of Personnel Management, said during a Senate hearing that the contractor USIS is being investigated and that the company performed a background investigation of Edward Snowden.

McFarland also told lawmakers that there may have been problems with the way the background check of Snowden was done, but McFarland and one of his assistants declined to say after the hearing what triggered the decision to investigate USIS and whether it involved the company’s check of Snowden.

‘‘To answer that question would require me to talk about an ongoing investigation. That’s against our policy,’’ Michelle Schmitz, assistant inspector general for investigations, told reporters after the hearing. ‘‘We are not going to make any comment at all on the investigation of USIS.’’

USIS, based in Falls Church, Va., did not immediately respond to a request for comment.

Senator Claire McCaskill, Democrat of Missouri, said she and her staff have been told that the inquiry is a criminal investigation related ‘‘to USIS’s systemic failure to adequately conduct investigations under its contract’’ with the Office of Personnel Management.

McCaskill said that USIS conducted a background investigation in 2011 for Snowden, who worked for government contractor Booz Allen Hamilton. Snowden says he is behind the disclosures about the NSA’s collection of Americans’ phone records and Internet data from US Internet companies.


‘‘We are limited in what we can say about this investigation because it is an ongoing criminal matter,’’ McCaskill said. ‘‘But it is a reminder that background investigations can have real consequences for our national security.’’

A background investigation is required for federal employees and contractors seeking a security clearance that gives them access to classified information.

Of the 4.9 million people with clearance to access ‘‘confidential and secret’’ government information, 1.1 million, or 21 percent, work for outside contractors, according to a January report from the Office of the Director of National Intelligence.