Among the questions for investigators is how a contract employee at a distant NSA office was able to obtain a copy of an order from the Foreign Intelligence Surveillance Court, a highly classified document that would presumably be sealed from most employees and of little use to someone in his position.
A former senior NSA official said that the number of agency officials with access to such court orders was "maybe 30 or maybe 40."
Snowden's whereabouts were unknown Monday, and it was unclear whether U.S. officials had sought to interview him or have him apprehended by officials in Hong Kong, where he had taken refuge.
Administration officials said they were working to confirm that Snowden leaked the documents and build a case without relying on his admissions in his interview with the Guardian. Investigators also need to determine whether anyone else was involved in disclosing the information to reporters, officials said.
FBI agents are interviewing Snowden's family and associates, said officials who spoke on the condition of anonymity to discuss the investigation.
Snowden, who said he leaked top-secret documents to expose abuse and not to cause damage, told the Guardian that he had "full access to the rosters of everyone working at the NSA, the entire intelligence community, and undercover assets all around the world, the locations of every station we have, what their missions are and so forth."
Officials questioned some of Snowden's assertions, saying several claims seemed exaggerated. Among them were assertions that he could order wiretaps on anyone from "a federal judge to even the president."
"When he said he had access to every CIA station around the world, he's lying," said a former senior agency official, who added that information was so closely compartmented that only a handful of top-ranking executives at the agency could access it.
One former NSA official said the agency employs layers of security to scrutinize employees, including keystroke-monitoring systems to identify potential breaches or unwarranted searches of NSA databases.
Joel Brenner, a former NSA inspector general, said any investigation needs to focus on how Snowden "had access to such a startling range of information."
One administration official said it was too early to determine how the United States would try to take custody of Snowden. Officials may simply try to see if authorities in Hong Kong will deport him.
In Hong Kong, Snowden checked out of a hotel Monday where he was thought to have been staying.
Some in Hong Kong said the semiautonomous jurisdiction may not offer Snowden the protection he seeks. "Hong Kong is definitely not a safe harbor for him," said Regina Ip, a lawmaker who noted the United States and Hong Kong have a strong record of cooperating on extradition.
Hong Kong has its own legislative and legal systems but ultimately answers to Beijing.
Given the touchy nature of China's relationship with the United States and Hong Kong, experts said the China was likely to stay in the background with this case.