U.S. National Security Agency whistleblower Edward Snowden, an analyst with a U.S. defence contractor, is seen in this still image taken from a video during an interview with the Guardian in his hotel room in Hong Kong June 6, 2013. (Reuters)

Before the world knew his name, 29-year-old Edward Snowden drafted a note of explanation.

He had worked for the CIA and as a contractor for the NSA, he wrote, and had lived a “comfortable and privileged life.” But he was also deeply uncomfortable with the knowledge that had already been afforded to him in his brief career — knowledge about the U.S. surveillance that officials said they were carrying out to keep America safe.

“As I advanced and learned the dangerous truth behind the U.S. policies that seek to develop secret, irresistible powers and concentrate them in the hands of an unaccountable few, human weakness haunted me,” Snowden wrote in the note, which would accompany the first documents he leaked. “As I worked in secret to resist them, selfish fear questioned if the stone thrown by a single man could justify the loss of everything he loves.

“I have come to my answer.”

Snowden, who identified himself Sunday as the main source behind recent disclosures of sweeping government surveillance programs, worked for years inside the U.S. intelligence community. As he did so, he said, he became disillusioned with American government policies.

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. Courtesy of Laura Poitras and Glenn Greenwald. (Nicki Demarco/Courtesy of Laura Poitras and Glenn Greenwald)

In an interview, he told The Washington Post that he could not “recall a single moment” in which his desire to violate his oath to protect top-secret information coalesced into the final decision to reveal that information publicly.

“It was more of a slow realization that presidents could openly lie to secure the office and then break public promises without consequence,” he said.

According to campaign finance reports, Snowden made a $250 donation to Ron Paul’s 2012 presidential campaign in March of that year, and gave another $250 in May. Paul has been a critic of excessive government intrusion.

Snowden, a soft-spoken “infrastructure analyst” with ties to the Washington area, said he advanced in the intelligence world through his understanding of computer programming and the Internet, though he has no visible Internet presence himself.

With wire glasses, short, dark hair and a thin goatee, he maintains an academic look. Yet he never completed his coursework at a community college in Maryland, only later obtaining his GED — an unusually light education for someone who would advance in the intelligence ranks.

For the past several months, Snowden was stationed in Hawaii, working as an NSA contractor for the firm Booz Allen Hamilton. It was there, at the NSA offices, he told the Guardian newspaper, that he copied the last set of documents he intended to disclose, told his NSA supervisors he needed time off for treatment for epilepsy, and boarded a flight to Hong Kong.

A storm has followed him. The Obama administration said Sunday that the NSA has asked the Justice Department to investigate the leak. Booz Allen condemned its erstwhile employee, and said Snowden had worked for them for less than three months.

“News reports that this individual has claimed to have leaked classified information are shocking, and if accurate, this action represents a grave violation of the code of conduct and core values of our firm,’’ the company said, vowing to work closely with authorities in the investigation.

Snowden, who says he is a former undercover CIA technical assistant, said he was undaunted. He also waded into the political debate he has helped trigger, expressing particular disappointment with President Obama and his administration, which he accused of failing to sufficiently investigate officials in the George W. Bush administration for earlier surveillance efforts.

“Excusing the prior administration from investigation wronged the public,” he said in a live, encrypted chat Sunday afternoon moments after his identity was made public. “It set an example that when powerful figures are suspected of wrongdoing, releasing them from the accountability of law is ‘for our own good.’ That’s corrosive to the basic fairness of society.”

The Guardian, which revealed Snowden’s identity Sunday at his request, started the cascade of national security revelations last week by reporting on the existence of a program that collects data on all phone calls made on the Verizon network. Later in the week, The Post and the Guardian reported the existence of a sep­arate program, code-named PRISM, that collects the Internet data of foreigners from major Internet companies.

Snowden has been working at the NSA for the past four years as an employee of several outside contractors. In addition to Booz Allen Hamilton, the Guardian reported that Snowden also worked for Dell, and it quoted him as saying he had “a very comfortable life,” with a salary that reached $200,000.

Snowden lived most recently in Hawaii, where a real estate agent, Kerri Jo Heim of Century 21, said she is now selling the house he rented. The three-bedroom house is in Waipahu, a short drive from the beach, and is now listed for $550,000. Snowden told the Post that he had a girlfriend with whom he lived in Hawaii.

Snowden and his girlfriend were strikingly standoffish while living in a home in the residential Royal Kunia neighborhood of Waipahu, and seemed to go out of their way to avoid conversations with passers-by, neighbor Carolyn Tijing said in a telephone interview. Tijing said that her husband went to introduce himself to Snowden and his girlfriend shortly after they moved across the street from the Tijings but that Snowden declined to exchange any pleasantries.

“It was a no-go, no conversation at all,” she said. “He just said ‘Fine’ and walked straight into his house. We thought they were just really anti-social.”

Carolyn Tijing said that the couple had erected a wall of boxes floor to ceiling inside their garage that blocked anyone’s view from the street into the two-car garage and that they always kept their cars parked in the driveway.Tijing said that she never saw anyone visit the home but that her college-aged son had seen several people stop by at late hours, between midnight and 2 a.m. Those visitors would arrive by car, stand in the driveway for a few minutes and exchange a few words with Snowden, then depart, Tijing said her son told her.

About four weeks ago, Snowden and his girlfriend apparently departed, Tijing said. The wall of boxes in the garage was gone, and a handyman arrived to clean the house. “One day they were here, the next they were gone,” Tijing said. “We never saw them leave.”

Numerous family members did not respond to phone calls and e-mails Sunday.

In the note accompanying the first document he gave The Post and others, Snowden wrote: “I understand that I will be made to suffer for my actions,” but “I will be satisfied if the federation of secret law, unequal pardon and irresistible executive powers that rule the world that I love are revealed even for an instant.”

Public records show residences for Snowden in the North Carolina cities of Elizabeth City and Wilmington and that his family later moved to Ellicott City, Md. — near the NSA headquarters at Fort Meade. Public records say his mother is a federal government employee: a deputy clerk for the U.S. District Court in Maryland.

A neighbor near the mother’s home said Sunday night that she recognized Snowden from photographs but had not seen him for many years. The neighbor confirmed that his mother has epilepsy and uses a service dog.

In 2003, according to the Guardian, Snowden enlisted in the U.S. Army, intending to join the Special Forces and fight in Iraq. But he was discharged after breaking both legs in a training accident.

His first NSA job was as a security guard for one of the agency’s covert facilities at the University of Maryland. Then he began working in IT security for the CIA, he says, with his understanding of the Internet and programming helping him overcome his lack of a high school diploma.

The CIA stationed him with diplomatic cover in Geneva in 2007, he said, which gave him widespread access to classified documents. Snowden said that access, along with his nearly three years around CIA officers, made him begin questioning U.S. government policies about surveillance.

After leaving the CIA in 2009 to work for a private contractor, he said, he was based at an NSA facility in Japan.

Snowden said he admires other accused leakers of government secrets, such as Pfc. Bradley E. Manning — who is accused of leaking classified documents to the anti-secrecy group WikiLeaks — but considers himself different.

“I carefully evaluated every single document I disclosed to ensure that each was legitimately in the public interest” he told the Guardian. “There are all sorts of documents that would have made a big impact that I didn’t turn over, because harming people isn’t my goal. Transparency is.”

Julie Tate, Peter Finn, Peter Hermann and Carol D. Leonnig contributed to this report.