(CNN) -- [Breaking news update at 10:07 a.m. Tuesday]
Authorities are preparing charges against suspected NSA leaker Edward Snowden, a law enforcement official told CNN on Tuesday.
The source said that a filing is not imminent.
[Original story, posted at 7:44 a.m. Tuesday]
NSA leak journalist says more revelations on their way
(CNN) -- As U.S. federal agents build a case against the contractor who exposed controversial electronic surveillance programs by the National Security Agency, one of the journalists who has been working with him says more secrets are set to be revealed soon.
"There are extremely invasive spying programs that the public still does not know about that the NSA regularly engages in or other capabilities that they're developing," said Glenn Greenwald, a columnist for the Guardian, the British newspaper that broke the first story based on secret NSA documents.
"We are working on stories right at this moment that we think are very valuable for the public to know that don't in any way harm national security but that shine a light on this extremely secretive though momentous agency," he said in an interview with CNN's Christiane Amanpour on Monday.
Greenwald received the documents from Edward Snowden, a 29-year-old employee at the computer consultant Booz Allen Hamilton, a contractor for the U.S. electronic intelligence agency.
Snowden told 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."
He 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 Snowden's home and computers and seeking interviews with his girlfriend, relatives, friends and co-workers.
The leaker's exact whereabouts are unclear at the moment.
Snowden checked out of a Hong Kong hotel where he had been staying on Monday but remains in the semiautonomous Chinese territory, Ewen MacAskill, one of the Guardian journalists who worked with him, said Tuesday.
His disclosures have 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.
Civil liberties advocates say the measures are an unacceptable intrusion into citizens' privacy. But 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."
Snowden's actions have 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."
A petition filed on the White House website Sunday, calling for the Obama administration to issue Snowden a complete pardon, had collected nearly 43,000 signatures by early Tuesday.
Uncertainty over next move
Despite Snowden's insistence that he didn't want the story to become about him, a great deal of attention remains focused on figuring out where he is and what will happen next.
He 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."
Legal experts say Hong Kong's extradition treaty with the United States could make it hard for Snowden to successfully fight any proceedings against him unless he is able to prove, for example, that any charges against him are politically motivated.
Although Hong Kong is part of communist-ruled China, the former British colony has a separate system of government that allows a free press and tolerates political dissent.
Patricia Ho, a lawyer with Daly & Associates in Hong Kong whose firm has handled asylum and refugee claims, said that given Hong Kong's lackluster track record on granting asylum, she was surprised Snowden had lauded the territory for its commitment to civil liberties.
"Within China itself, Hong Kong has better civil liberties but I couldn't see the Hong Kong government granting him asylum given their present practices," she said.
Advice from Assange
Snowden has told the Guardian he hopes to seek asylum, potentially in Iceland 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.
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.
Traveling to another country could become difficult for Snowden if U.S. authorities issue an Interpol "red notice" against him, according to Don Borelli, a former FBI agent and U.S. legal attache overseas.
"Many countries recognize an Interpol red notice as kind of a universal arrest warrant," he said.