(CNN) -- The White House said Monday it welcomes a debate over the electronic surveillance programs exposed by a National Security Agency contractor, even as federal agents began building a case against the self-proclaimed leaker.
Edward Snowden told the British newspaper the Guardian that he left behind his family and a six-figure job in Hawaii to reveal the extent of the NSA's collection of telephone and Internet data, which he called "an existential threat to democracy." The 29-year-old worked for computer consultant Booz Allen Hamilton, a contractor for the U.S. electronic intelligence agency.
Snowden said he expects to be prosecuted for the leak, and a federal law enforcement official said Monday that FBI agents have begun an investigation by searching the 29-year-old's home and computers and seeking interviews with his girlfriend, relatives, friends and co-workers.
Snowden outed himself Sunday in the Guardian, which began publishing details of his revelations last week. He said he expects to be prosecuted but acted in hopes of ending what he called an excessively intrusive system, the Guardian reported.
"The government has granted itself power it is not entitled to. There is no public oversight. The result is people like myself have the latitude to go further than they are allowed to," Snowden told the paper. He was also the source for stories on the NSA's operations in The Washington Post, that newspaper reported.
Don Borelli, a former FBI agent and U.S. legal attache overseas, said computer forensics will be an important element of any case against Snowden, who took off for Hong Kong before the stories were published.
"You need to corroborate what he said," Borelli told CNN. "You need to be able to prove the elements of a crime."
Snowden checked out of his Hong Kong hotel Monday but remains in the semiautonomous Chinese territory, said Ewen MacAskill, the Guardian's Washington bureau chief.
MacAskill told CNN that Snowden planned his disclosure and his getaway in great detail -- "but this next phase, the phase we're in now, he was almost vague about it," MacAskill said. "I don't think he actually knew or even cared that much. His main objective was to get the information about the level of surveillance out into the public domain and then beyond that, he didn't care."
Snowden's revelations fueled new debate about the U.S. government's collection of records of domestic telephone calls and overseas Internet activity in the global hunt for terrorists and criminals. Supporters of the programs say they are legal and have yielded evidence that has helped put terror plotters in prison, though many of the details remain classified.
Obama administration officials and leaders of the intelligence committees in Congress say the program undergoes periodic review by all three branches of government, and that the content of Americans' calls is not being monitored.
White House spokesman Jay Carney said Monday the measures are a necessary middle way between total privacy and unacceptable threat. He said President Barack Obama would be willing to consider changes should a national debate show the public wants them -- but he wryly noted, "This is not the manner by which he hoped to have the debate."
"What I can tell you is that the programs are judged by the president and by his national security team to be necessary and effective. They are also accorded oversight by all three branches of government, as is appropriate, and it is also the case that these programs and the general principle about finding the balance between our security interests and our need and desire for privacy is something that we should constantly engage in.
U.S. Rep. Peter King, chairman of the House Homeland Security Committee's Subcommittee on Counterterrorism and Intelligence, called Snowden "a defector" who should be turned over to the United States with an eye toward harsh prosecution.
"This person is dangerous to the country," King said on CNN's "Starting Point" on Monday.
Sen. Bill Nelson, D-Florida, fumed that Snowden committed "an act of treason," while Senate Intelligence Committee Chairwoman Dianne Feinstein questioned whether his access to government secrets was as extensive as he claims.
"I listened carefully to what he said, and what he said is, you know, 'I can get access to where every asset is in the world and their stations and their missions,' and I've been told that isn't possible," said Feinstein, D-California.
She added, "I don't really have any way of knowing how adept he is in the computer world. ... I can't say he's overstating anything in these programs. Maybe he's overstating his prowess."
But Glenn Greenwald, the Guardian columnist who was the lead author on last week's stories, told CNN's "The Lead" that Snowden has revealed secrets that were being kept only from American citizens.
"Terrorists already know the U.S. government tries to surveil their communications," Greenwald said. "Nothing that we revealed helps, quote-unquote, the terrorists. All we did was tell our fellow citizens of the United States and around the world the extent and capabilities of how vast the surveillance state is and the reasons why it needs scrutiny and accountability. And the only things we damaged are the reputation of American political officials, not national security."
Greenwald said he knew "generally" where Snowden has gone, but added, "I'm not going to disclose information about his whereabouts. He's capable of doing that himself if he wants to."
Extradition for Snowden?
A major question is whether Hong Kong, where Snowden fled, would extradite him to face charges in the United States.
Although Hong Kong is part of communist-ruled China, the former British colony has a separate ruling system that allows a free press and tolerates political dissent. Hong Kong's extradition treaty with the United States has exceptions for political crimes and cases when handing over a criminal suspect would harm the "defense, foreign affairs or essential public interest or policy" of either party.
"I think he looked around, this seemed the safest bet," said MacAskill.
Snowden hopes to get asylum, he added, with Iceland his first choice because of the way it dealt with WikiLeaks, a group that facilitates the anonymous leaking of secret information through its website. The group reportedly once operated from there.
Kristin Arnadottir, Iceland's ambassador to China, said Icelandic law requires asylum applications to be made from inside the country.
WikiLeaks founder Julian Assange, bottled up in the Ecuadorian Embassy in London since last June, said Snowden should be looking to the southeast, not northwest.
"I would strongly advise him to go to Latin America," Assange told CNN's "Anderson Cooper 360" on Monday night. "Latin America has shown in the past 10 years that it is really pushing forward in human rights. There's a long tradition of asylum."
Assange sought refuge in the Ecuadorian mission to avoid extradition to Sweden, where he is wanted for questioning over allegations that he raped one woman and sexually molested another. He has repeatedly said the allegations in Sweden are politically motivated and tied to the work of his website.
Assange has said he fears Sweden will transfer him to the United States, where he could face the death penalty if he were charged with and convicted of a crime. Though no U.S. charges are pending, WikiLeaks source Bradley Manning is being court-martialed on charges that he aided U.S. enemies by leaking documents he obtained as a U.S. Army intelligence analyst.
If Snowden is charged and brought back to the United States, the Progressive Change Campaign Committee started a campaign to raise funds for his legal defense. The group promotes progressive causes and candidates, its website says.
Snowden's revelations began Wednesday when the Guardian published a top secret court order demanding that Verizon Business Network Services turn over details of phone calls published from April 25 to July 19. Intelligence officials later confirmed the program, which analysts say likely covers all U.S. carriers.
On Thursday, the Guardian and the Post disclosed the existence of PRISM, a program they said allows NSA analysts to extract the details of people's online activities -- including "audio and video chats, photographs, e-mails, documents" and other materials -- from computers at Microsoft, Google, Apple and other Internet firms.
Intelligence officials similarly confirmed that program's existence, but said it targets only overseas residents who are not U.S. citizens.
On Friday, Obama said he entered office skeptical of such programs but decided to reauthorize them after a thorough vetting and the addition of unspecified additional safeguards. He called them only "modest encroachments on privacy" that help thwart terror attacks.
Before joining Booz Allen Hamilton, which provides support technology and computer support to the government, Snowden worked for the CIA, he told the newspaper.
He told the Guardian he worked for the consulting firm in Hawaii, holding down a $200,000-a-year job that gave him easy access to a vast trove of sensitive data.
"The NSA has built an infrastructure that allows it to intercept almost everything," he told the newspaper. "With this capability, the vast majority of human communications are automatically ingested without targeting. If I wanted to see your e-mails or your wife's phone, all I have to do is use intercepts. I can get your e-mails, passwords, phone records, credit cards."
In a statement released Sunday, Booz Allen Hamilton said Snowden had worked for the company for less than three months. The report that he had leaked American secrets was "shocking" and if true, "represents a grave violation of the code of conduct and core values of our firm," the company said.
Snowden's actions, while opposed by many, have also brought together some liberals and conservatives to hail him as a hero.
Liberal activist and filmmaker Michael Moore tweeted that Snowden is "HERO OF THE YEAR." Conservative commentator Glenn Beck, meanwhile, called Snowden a "patriot leaker" who could help America "regain her moral compass."
Daniel Ellsberg, who in the 1970s leaked the Pentagon Papers documents showing the government had lied about the progress of the Vietnam War, said Snowden had done the country an "enormous service." And in Congress, Democratic senators such as Ron Wyden of Oregon and Mark Udall of Colorado have warned about the dangers of excessive surveillance as vociferously as Sen. Rand Paul, a Republican from Kentucky.
On the other side, Feinstein joined Republicans such as Sen. Saxby Chambliss of Georgia and Rep. Mike Rogers in defending the surveillance programs.
They point to what they say are the program's successes, including charges against an Afghan-born Colorado man who pleaded guilty to conspiring to bomb targets in New York, and David Headley, who was accused of conducting advance surveillance for the Pakistani jihadists who attacked hotels and other targets in Mumbai, India, in 2008, killing 164 people. Both men pleaded guilty.
Rogers, the chairman of the House Intelligence Committee, told ABC's "This Week" that the "inflammatory nature" of the accusations doesn't fit with how the program actually operates.
"The instances where this has produced good -- has disrupted plots, prevented terrorist attacks -- is all classified," said Rogers, R-Michigan. "That's what's so hard about this."
CNN's Joe Johns, Carol Cratty, Jethro Mullen Tom Cohen, Michael Pearson, Brian Walker, Anjali Tsui and Elise Labott contributed to this report.