30-year-old American Edward Snowden, a contract employee at the National Security Agency, is the whistleblower behind significant revelations that surfaced in June 2013 about the US government's top secret, extensive domestic surveillance programmes. Snowden flew to Hong Kong from Hawaii in May 2013, and supplied confidential US government documents to media outlets including the Guardian.
Greenwald's passionate opinions become part of story
Whistle-blower Snowden took his revelations to a writer he knew had strong opinions, but some question the fairness of this kind of reporting
The man who claimed to leak state secrets on US government eavesdropping sought to break the story through a columnist for a Britain-based publication who has made no secret of his distaste for intrusions on privacy.
Edward Snowden brought his information to Glenn Greenwald of The Guardian instead of to The Washington Post, with which he had briefly corresponded. The case illustrates the passion an opinion-driven journalist can bring to a breaking news story; at the same time it raises questions about fairness.
Greenwald, author of three books in which he argues that the US government has trampled on personal rights in the name of protecting national security, wrote the original stories exposing the extent of the government's data collection.
Snowden sent an e-mail to Greenwald, who lives in Brazil, suggesting he set up a method for receiving and sending encrypted e-mails. Snowden even made a YouTube video for Greenwald, to take him step by step through the process of encryption.
Greenwald was unaware of the identity of the person offering the leaks, and was unsure whether they were genuine. He took no action. In March in New York, he received a call from filmmaker and journalist Laura Poitras, who convinced him he needed to take this more seriously.
In January, Snowden had reached out to Poitras, and they began a correspondence.
Greenwald and Snowden set up a secure communications system, and the first of the documents arrived, dealing with the National Security Agency's (NSA) secret Prism programme, which recruits the major internet companies to help with surveillance.
Greenwald flew to New York to talk to Guardian editors on May 31, and the next day he, Poitras and Ewen MacAskill, a Guardian reporter, flew to Hong Kong. Neither Greenwald nor Poitras knew what Snowden looked like.
Snowden had told him to go to a specific location on the third floor of the hotel and to ask loudly for directions to a restaurant. If all seemed well, the source would walk past holding a Rubik's Cube, he was told. They followed the directions. A man with a Rubik's Cube appeared.
They went to a room - which, Greenwald recalled, contained a large stuffed alligator - where Snowden made himself known to the pair.
Greenwald and Poitras were shocked the first time they saw the 29-year-old. "I had expected a 60-year-old, grizzled veteran - someone in the higher echelons of the intelligence service," Greenwald said. "I thought: 'This is going to be a wasted trip.'"
After an hour of listening to Snowden, though, Greenwald changed his mind. "I completely believed him," he said.
"What we disclosed was of great public interest, of great importance in a democracy, that the US government is building this massive spying apparatus aimed at its own population," Greenwald told the Morning Joe programme on the US cable channel MSNBC.
The topic is personal for Greenwald, 46. The former constitutional and civil-rights lawyer, educated at the New York University Law School, began the Unclaimed Territory blog in 2005 and wrote How Would a Patriot Act? a year later. The book criticised the administration of US president George W. Bush for its use of executive power.
Greenwald wrote a regular column for Salon for five years until joining The Guardian last year. He said he wanted to reach a more international audience, a desire that coincided with the news organisation's effort to expand its reach in the US market.
One programme he wrote about collects hundreds of millions of US phone records. The second programme takes in audio, e-mail and other electronic activities primarily by non-US nationals who use providers including Microsoft and Apple.
Greenwald described the collection of phone records as "rampant abuse, and it needs sunlight. That's why this person came forward and that's why we published our stories.
"The wall of secrecy behind which they operate is impenetrable and it is a real menace to democracy," said Greenwald, who won a 2010 Online Journalism Association award for his coverage of Bradley Manning, who is charged with giving secret US documents to WikiLeaks.
Greenwald's clear point of view does not necessarily weaken the story, said Jay Rosen, journalism professor at New York University and author of the Press Think blog.
"In many ways it strengthens it," he said. Greenwald has a clear stance on privacy and national security, but they are not partisan; he has criticised Democratic President Barack Obama and his Republican predecessor, Bush.
Journalists who have strong viewpoints are a tradition with a long history in the United States, Rosen said. "The fact that sources now may choose [outlets] on the basis of commitment is a fact, and journalists whose professional stance is no commitment may find themselves at a disadvantage," he said.
Greenwald's known feelings on the issue "does leave a little opening for critics", said Ellen Shearer, head of the national- security journalism initiative at Northwestern University in Illinois. There is always a risk that such passion can work against a journalist; some people would worry that facts contradictory to a predisposed belief could be overlooked.
To this point, Shearer said there had been little pushback on the facts, with the debate primarily about whether the information should be published.
US intelligence officials are investigating the leak and its impact on its programmes. Director of National Intelligence James Clapper called the revelation of the intelligence-gathering programmes reckless and said it had done "huge, grave damage".
The Guardian took care not to publish material that may help other countries improve their eavesdropping or could put the lives of covert agents at risk, Greenwald said.
"We've published these things they marked 'top secret' that don't actually harm national security but conceal what they've done from the public," he said.
But Pete King, Republican chairman of the US House of Representatives' Subcommittee on Counterterrorism and Intelligence, has called for prosecution of Greenwald.
Greenwald said he had spoken to lawyers about whether he is at legal risk. "I have spoken with lawyers about representing me in the event that I need one."
He said he had not dismissed the possibility that he really may face prosecution, given the Obama administration's aggressive prosecution of leak cases. "It would be irrational for me to dismiss the possibility."
Greenwald added, however, that the threat of prosecution would only encourage him to continue: "It's not going to deter me or limit me or constrain me in any way from exercising my core [US constitutional] rights" to free speech.
Additional reporting by The Guardian, The New York Times, The Washington Post