U.S. defense contractor Edward Snowden discusses his motivation behind the NSA leak and why he is revealing himself as the whistleblower behind the major story. (Nicki Demarco/Courtesy of Laura Poitras and Glenn Greenwald)

Counterintelligence investigators are scrutinizing how a 29-year-old contractor who said he leaked top-secret National Security Agency documents was able to gain access to what should be highly compartmentalized information, according to current and former administration and intelligence officials.

Edward J. Snowden worked as a systems administrator at an NSA Threat Operations Center in Hawaii, one of several such facilities that are tasked with detecting threats to government computer systems. He has previously worked for the CIA, U.S. officials said.

Snowden leaked documents to The Washington Post and Britain’s Guardian newspaper on distinctly different operations: the NSA’s collection of data from U.S. phone call records and its surveillance of online communications to and from foreign targets.

Investigators are “working with the NSA and others around the intelligence community to understand exactly what information this individual had access to, and how that individual was able to take that information outside the community,” a senior U.S. intelligence official said.

Among the questions is how a contract employee at a distant NSA satellite 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 is “maybe 30 or maybe 40. Not large numbers.”

Snowden’s exact 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 Monday that they are working to confirm that Snowden leaked the documents and build a case against him without relying on his admissions in his video 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 to the United States, 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 in his interview with the Guardian, saying that several of his 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 is so closely compartmented that only a handful of top-ranking executives at the agency could access it.

Current and former administration officials were flummoxed by Snowden’s claim that he was authorized to access the orders from the Foreign Intelligence Surveillance Court.

The order probably would have been accessible to the NSA general counsel’s office, the compliance office that deals with the court, and the operational arm carrying out the collection, former officials said.

One former NSA official said the NSA 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.”

“The spy you want in an organization may not be the executive assistant to the secretary of state; it may be the guy in the bowels of the IT department because he has system-administrator privileges and because that person is also in a position to insert malware into your system to facilitate remote access,” Brenner said.

Further information about Snowden’s personal and professional life was scant Monday.

His mother, who lives in Maryland, and his father, who retired from the U.S. Coast Guard in 2009 and lives in the Lehigh Valley area of Pennsylvania, would not speak about their son. The parents divorced in 2001, about two years after Snowden left high school.

Snowden joined the Army in 2004 and trained for the Special Forces for four months before leaving the service without finishing, an Army spokesman said. Snowden said he departed because he had broken his legs in a training accident and was discharged. An Army spokesman did not confirm that account, saying only that “he did not complete any training or receive any awards.”

Snowden said he then worked at the University of Maryland as a security guard at a secret NSA facility near the College Park campus. University spokesman Brian Ullmann confirmed that in 2005, Snowden worked for less than a year as a “security specialist” for the school’s Center for Advanced Study of Language. The university-affiliated center, founded in 2003, is not a classified facility.

Snowden said that after his stint at U-Md., the CIA hired him to work on technology security. The agency has declined to comment on his employment.

Snowden said he left the CIA in 2009 to work for the NSA through two private contractors, first at a Dell computers operation in Columbia, Md., and then at Booz Allen Hamilton. Dell spokesman Scott Radcliffe would not confirm whether Snowden worked there, saying that “the Justice Department has asked us to refer all questions to them.” The government contractor confirmed that Snowden had worked there for the past three months in an office in Hawaii. He flew to Hong Kong after telling his supervisor in Hawaii that he needed medical treatment for epilepsy.

One administration official said it is too early to determine how United States will attempt to take custody of Snowden. Officials may simply attempt to see if the authorities in Hong Kong will deport him and avoid the need for a full extradition procedure.

“Ultimately, a lot of these cases get resolved through something other than formal extradition,” said Shane Kadidal, a lawyer at the Center of Constitutional Rights in New York.

In Hong Kong, Snowden checked out of a hotel Monday where he was thought to be staying.

Some in Hong Kong said the semi-autonomous jurisdiction may not offer Snowden the protection he hopes for. “Hong Kong is definitely not a safe harbor for him,” said Regina Ip, a lawmaker and chairman of the New People’s Party.

Hong Kong has its own legislative and legal systems but ultimately answers to Beijing, under the “one country, two systems” arrangement, established when oversight of Hong Kong was transferred from the British to the Chinese in 1997. The extradition treaty between Hong Kong and the United States was established at the time of the British-Chinese handover, because the treaty needed the blessing of Hong Kong’s new sovereign ruler, the Chinese government.

The treaty says that Hong Kong can refuse to transfer a suspected criminal to the United States if giving up the person “implicates” the “defense, foreign affairs or essential public interest or policy” of the People’s Republic of China.

Given the touchy nature of China’s relationship with the United States and Hong Kong, however, experts said the Chinese government is likely to stay in the background with Snowden’s case.

Ip noted that the United States and Hong Kong have a strong record of cooperation on the extradition of fugitives.

Top officials from the Justice Department, the FBI and the National Security Agency will appear Tuesday in front of House members to discuss the NSA’s surveillance efforts and the fallout from the leaks. The Senate will hold a similar closed hearing on Thursday.

Administration officials stressed Monday that they have briefed Congress repeatedly on surveillance programs

Jia Lynn Yang in Hong Kong, Liu Liu in Beijing, Sari Horwitz, Julie Tate, Barton Gellman, Jenna Johnson, Peter Hermann, Marc Fisher and Carol D. Leonnig contributed to this report.