Mark Felt, the mysterious Deep Throat source who helped Washington Post reporters Bob Woodward and Carl Bernstein crack the Watergate scandal that brought down President Richard Nixon, has died at age 95.
Mr Felt suffered from congestive heart failure but the exact cause of his death at home on Thursday was not immediately known, said the Press Democrat newspaper in Santa Rosa, California, 90 kilometres north of San Francisco.
In its report on Mr Felt's death, the New York Times called him "the most famous anonymous source in American history."
Mr Felt, the No. 2 official at the FBI when the Watergate case broke, kept his role in the story a secret for 30 years. Only in 2005, at age 91, was his part made public in an article in Vanity Fair magazine written by Mr Felt's family lawyer.
"I'm the guy they used to call Deep Throat," Mr Felt told attorney John O'Connor.
For years, people had speculated and argued about the identity of Deep Throat, whose name was derived from the title of a popular pornographic movie.
Vanity Fair scooped Mr Woodward and Mr Bernstein, who had promised not to reveal the name of the star source of their 1974 stories until after his death. But within a day of Mr Felt's unveiling, Mr Woodward wrote of his relationship with Mr Felt.
Mr Woodward said he turned to Mr Felt after he and Mr Bernstein wrote about the break-in at the offices of the Democratic National Committee at the Watergate complex in Washington.
"This was the moment when a source or friend in the investigative agencies of government is invaluable," Mr Woodward wrote in the Post. "I called Mr Felt at the FBI... It would be our first talk about Watergate."
Mr Woodward said Mr Felt told him the Watergate case was going to "heat up." He abruptly hung up but then started to provide guidance on the story, said Mr Woodward.
Following a complicated routine, Mr Felt and Mr Woodward would arrange to meet in an underground garage, with Deep Throat corroborating information the Post reporters had gleaned from other sources and outlining a government conspiracy.
"Mr Felt believed he was protecting the bureau by finding a way, clandestine as it was, to push some of the information from the FBI interviews and files out to the public, to help build public and political pressure to make Nixon and his people answerable," wrote Mr Woodward.
"He had nothing but contempt for the Mr Nixon White House and their efforts to manipulate the bureau for political reasons."
Reporting by the Post and other news organisations on the White House's involvement in the Watergate break-in and other political "dirty tricks" forced Mr Nixon's resignation in 1974.
More than 30 officials would ultimately plead guilty or be convicted, including Attorney General John Mitchell, who served 19 months for conspiracy, obstruction of justice and perjury.
Mr Felt repeatedly denied he was Deep Throat, even though his position at the FBI made him an obvious candidate, and Mr Nixon himself suspected Mr Felt of leaking to the media.
A character patterned on Mr Felt showed up in the book and movie All The President's Men, an account by Mr Bernstein and Mr Woodward of their Watergate reporting. Played by Hal Holbrooke, the Deep Throat character was seen in the shadows.
This portrayal was true to life, wrote Mr Woodward, because Mr Felt insisted on anonymity and was concerned that phone calls might be tapped.
Last month, Mr Woodward and Mr Bernstein visited Mr Felt in Santa Rosa. It was Mr Bernstein's first meeting with the famous source, who dealt only with Mr Woodward during the Watergate days.