In a weekend interview with NBC News, Mr. Clapper warned that the revelations could create serious risks to national security. “We’re very, very concerned about it,” he said. “For me, it is literally — not figuratively — literally gut-wrenching to see this happen, because of the huge, grave damage it does to our intelligence capabilities.”

Mr. Snowden, a native of North Carolina, told The Guardian that he signed up in 2003 for an Army Special Forces training program because he wanted to fight in Iraq.

“I felt like I had an obligation as a human being to help free people from oppression,” he said.

But he said he had quickly become disillusioned with the military.

“Most of the people training us seemed pumped up about killing Arabs, not helping anyone,” he said.

After breaking his legs during a training accident, Mr. Snowden was discharged from the Army and took a job as a security guard at an N.S.A. secret facility on the University of Maryland’s campus, according to The Guardian, which said it had confirmed his story.

Despite not having a high school degree, he was later hired by the C.I.A. to work on information technology security, serving in Geneva. In 2009, he joined the N.S.A. as a contractor at a facility in Japan, where, he said, he watched “as Obama advanced the very policies that I thought would be reined in.”

Most recently, Mr. Snowden has been part of a Booz Allen team working at an N.S.A. facility in Hawaii. Three weeks ago, he made final preparations to disclose the classified documents, The Guardian said. It said he had copied the documents and told a supervisor that he needed to take a few weeks off to deal with medical problems. He then flew to Hong Kong.

While it was not clear whether Mr. Snowden had remained in Hong Kong, if he had, his presence could complicate any possible American effort to extradite him for prosecution. A British colony until its return to China in 1997, Hong Kong retains autonomy from the mainland in its immigration system and its rule of law. Hong Kong has an independent immigration system, but it is part of China for purposes of foreign policy.