"Follow the money". This famous phrase has inspired generations of investigative reporters, but its author managed to evade detection for 30 years.
It was one of America's greatest mysteries: Who was the anonymous source who had leaked information about the Watergate scandal that led to the resignation of US President Richard Nixon in 1974?
Mark Felt was at the centre of one of the longest-running mysteries in journalism
Mark Felt, a former deputy head of the FBI, revealed in 2005 that it was he who made the suggestion that led to the discovery of the link between the burglary at the Democratic National Committee HQ in Washington's Watergate complex in June 1972, and the financing of Nixon's re-election campaign.
For decades, the informant was known only as Deep Throat. He was the shadowy, chain-smoking character played by Hal Holbrook in the hit movie All the President's Men, starring Robert Redford and Dustin Hoffman.
Mr Felt, who was responsible for investigating the burglary, had figured prominently in the 30-year guessing game about Deep Throat's identity.
He repeatedly denied that he was the source who met Washington Post reporters Bob Woodward and Carl Bernstein in underground car parks to provide clues to the scandal.
But in 2005, in an article written by his friend and lawyer John O'Connor for the US magazine Vanity Fair, he admitted: "I'm the guy they used to call Deep Throat."
Carl Bernstein and Bob Woodward said Mr Felt took a "monumental risk"
According to reports, he had lived for decades in the belief that he betrayed his FBI badge by disclosing government secrets.
"Mark felt that he was somehow a dishonourable guy, an FBI agent who was disloyal, who leaked when he shouldn't have leaked. He kept saying an FBI agent doesn't do this," Mr O'Connor told US media in 2005.
Mr Felt's family only learned of his secret three years earlier and, according to Mr O'Connor, they helped convince him that he "was a hero".
"After talking to him for two to three years, probably for the last six to nine months, he was really convinced he was a hero. He knows he did the right thing. He knows he had to breach his code of ethics to save the country."
Mr Felt's son, Mark Junior, told Vanity Fair: "He would not have done it if he didn't feel it was the only way to get around the corruption in the White House and Justice Department. He was tortured inside, but never would show it."
While many praised Mr Felt's role as an informant, some turned on the former FBI man. The former Nixon speechwriter and presidential hopeful, Pat Buchanan, described Mr Felt a traitor.
Mark Felt spent his latter years living in Santa Rosa, California. He died at the age of 95 at a hospice close to his home.
The former FBI official, described by Mr Woodward as a dashing, grey-haired figure, has a few possible motives for doing what he did. One was personal - anger at being passed over for director after the death of J Edgar Hoover. The other was concern that the White House was sabotaging the FBI's Watergate investigation.
In his memoir, The FBI Pyramid, Mr Felt wrote that Patrick Gray, who succeeded Hoover, was "sharing all the Bureau's knowledge with the White House staff". He wrote that he "felt they had neutralised the FBI".
Nixon was forced to resign in 1974
"For me, as well as for all the agents who were involved, it had become a question of our integrity," Mr Felt wrote. "We were under attack for dragging our feet, and as professional law enforcement officers, we were determined to go on."
Mr Gray was never confirmed as FBI director, and in 1973 William D Ruckelshaus took over the bureau. Mr Felt left the bureau later that year.
He had had a long career with the FBI and, according to reports, was seen as a model official. Quoted in the Washington Post, Harry Brandon, who retired from the FBI as deputy assistant for counterintelligence and counterterrorism, described him as: "Straight. Very honest. Very straight."
Mr Felt was born in Idaho in 1913. He attended the University of Idaho and George Washington University Law School, before joining the FBI in January 1942.
During World War II he worked in the agency's espionage section. According to the Washington Post, it was here that he learned counter-intelligence tricks that became part of his relationship with their reporters: a flowerpot on Mr Woodward's balcony would indicate that the reporter required a meeting, while a clock face inked on the reporter's daily New York Times would reveal the time Mr Felt would be waiting in the car park.
Mr Felt steadily rose through the FBI's ranks and by the early 1970s was one of the bureau's top officials.
In 1978, he was involved in one of the most prominent cases of high-ranking FBI officials being the target of criminal charges.
He was indicted with another official, Edward Miller, on charges of authorising illegal break-ins during the Nixon administration on friends of members of the radical anti-Vietnam war movement, the Weather Underground.
They were convicted in 1980 but pardoned a few months later by President Ronald Reagan. The president said they had "acted on high principle to bring an end to the terrorism that was threatening our nation".