Now at age 29, Snowden has become known worldwide as the man responsible for exposing vast surveillance programs by the National Security Agency, one of the most secretive government agencies in the United States.
Snowden stepped from the shadows and admitted that he had exposed the US government's top-secret surveillance programs to Britain's Guardian newspaper and the Washington Post after working in Hawaii for a company under contract to the NSA.
Americans are debating whether he is a patriotic defender of civil liberties or the most unprincipled of traitors.
Snowden saw his role more clearly, saying the US government's powers of surveillance have grown so immense and intrusive that he felt compelled to denounce them, even at great personal cost. He could have remained anonymous but said his message would resonate more powerfully if he revealed his identity.
"The public needs to decide whether these programs and policies are right or wrong," Snowden told the Guardian in the 12-minute video introducing him to the world on Sunday.
Abandoning his life in Hawaii last month, Snowden went into hiding in Hong Kong, saying he feared he could be captured by the CIA, another foreign government or Asian organized crime gangs.
"That's a fear I'll live under for the rest of my life, however long that happens to be," he said in the video.
In his secretive dealings with the Washington Post, he took the codename Verax - Latin to describe a truth teller - the paper said.
"He's very intelligent, calm, (but) always scared that someone would knock on the door and he'd be taken away," said Ewen MacAskill, one of the Guardian journalists who worked on the story.
Snowden expressed some interest in seeking asylum in Iceland. He checked out of his hotel in Hong Kong on Monday and his whereabouts were not known.
In recent years, he had returned to the Washington suburbs of his youth, before taking his final assignment in Hawaii.
"He was quiet, shy, always walked around with his head down," said Joyce Kinsey, a neighbor in Ellicott City, Maryland, who said Snowden moved into an apartment there about three years ago and that his mother soon followed. He later moved to Hawaii.
"They are a nice family. I feel really, really sorry for his mother. She always left her curtains open and you could see right in. But now since all these reporters showed up, she's keeping the curtains closed. This whole neighborhood is shocked."
Little from Snowden's childhood could portend his future place alongside Daniel Ellsberg, who disclosed the so-called Pentagon Papers during the Vietnam War, and Bradley Manning, the US Army private on military trial for providing WikiLeaks with documents, as one of the most important leakers of US secrets.
As a youth in the Washington suburb of Fort Meade, Maryland, Snowden attended local schools, but dropped out of Arundel High School about halfway through his sophomore year, said school spokesman Bob Mosier.
Snowden's parents divorced when he was 18 and they lived in Crofton, Maryland, a planned community where many NSA employees and their families live.
Snowden told the Guardian he joined the military with the idea of aiding the US war effort in Iraq and to "help free people from oppression."
He lasted only four months after breaking both legs in a training exercise, he said. Pentagon records show he enlisted in the Army Reserve as a special forces recruit, entering in May 2004 and leaving four months later without completing his training.
Later he landed his first job at the University of Maryland working at a covert NSA facility near the campus, Snowden said. He then went to the CIA in information technology security, rising quickly because of his understanding of the Internet and his computer programming skills, he told the Guardian.
By 2007, the CIA had stationed him with diplomatic cover in Geneva, Switzerland, where he maintained computer network security, he told the Guardian.
His experience there and working alongside CIA officers gradually led him to question his own role in the government.
With time, Snowden told the Guardian, he could no longer live "unfreely but comfortably" as a well-paid infrastructure analyst for Booz Allen Hamilton, a company hired by the NSA to manage its surveillance system.
He said he was "willing to accept any risk" by revealing top secrets and left his live-in girlfriend behind.
Snowden's parents and sisters did not return phone calls or emails, nor did a 28-year-old woman named Lindsay A. Mills who lived at the same address with him in Maryland and Hawaii.
"For those of you that know me without my superhero cape, you can probably understand why I'll be refraining from blog posts for a while," Mills, a dancer, wrote in a blog post on Monday. "At the moment, all I can feel is alone. And for the first time in my life I feel strong enough to be on my own."
Among the telling details on the public record was Snowden's support for Ron Paul, the libertarian US politician who has run unsuccessfully for the Republican Party's presidential nomination.
Campaign finance records showed Snowden twice donated $250 to Paul's 2012 campaign, which was based largely on the principle that government has grown too meddlesome and intruded on personal freedom.
With little more information than that, sympathisers portrayed Snowden as a hero.
A fund started on the online fundraising platform Crowdtilt had raised more than $8,100 by Monday afternoon, saying the cash would support Snowden for any expenses, such as hotels and airfare. Separately, the Progressive Change Campaign Committee said it was raising money for his legal defense. An online petition asking the White House to pardon Snowden accumulated some 26,550 signatures.
There was an angry backlash as well. Some denounced him as an anti-American spy. One US counterterrorism official was worried by reports that Snowden handled or had access to CIA and NSA communications in Geneva and Japan, two vital listening posts.
"He might be young, but this is not exactly a low-level guy. He's privy to a lot. What scares me is what else he knows, and if the Chinese will get to him," said the official, who is regularly briefed on reports under the Foreign Intelligence Surveillance Act and spoke on condition of anonymity.
The official noted that the information Snowden leaked was "top secret," compared to lower-level "secret" information that Manning admitted he provided to WikiLeaks. "So this is a lot more damaging," the official said.
