W. Mark Felt, who was the No. 2 official at the F.B.I. when he helped bring down President Richard M. Nixon by resisting the Watergate cover-up and becoming Deep Throat, the most famous anonymous source in American history, died Thursday. He was 95 and lived in Santa Rosa, Calif.

His death was confirmed by Rob Jones, his grandson.

In 2005, Mr. Felt revealed that he was the one who had secretly supplied Bob Woodward of The Washington Post with crucial leads in the Watergate affair in the early 1970s. His decision to unmask himself, in an article in Vanity Fair, ended a guessing game that had gone on for more than 30 years.

The disclosure even surprised Mr. Woodward and his partner on the Watergate story, Carl Bernstein. They had kept their promise not to reveal his identity until after his death. Indeed, Mr. Woodward was so scrupulous about shielding Mr. Felt that he did not introduce him to Mr. Bernstein until this year, 36 years after they cracked the scandal. The three met for two hours one afternoon last month in Santa Rosa, where Mr. Felt had retired. The reporters likened it to a family reunion.

Mr. Felt played a dual role in the fall of Nixon. As a secret informant, he kept the story alive in the press. As associate director of the Federal Bureau of Investigation, he fought the president’s efforts to obstruct the F.B.I.’s investigation of the Watergate break-in.

Without Mr. Felt, there might not have been a Watergate — shorthand for the revealed abuses of presidential powers in the Nixon White House, including illegal wiretapping, burglaries and money laundering. Americans might never have seen a president as a criminal conspirator, or reporters as cultural heroes, or anonymous sources like Mr. Felt as a necessary if undesired tool in the pursuit of truth.

Like Nixon, Mr. Felt authorized illegal break-ins in the name of national security and then received the absolution of a presidential pardon. Their lives were intertwined in ways only they and a few others knew.

Nixon cursed his name when he learned early on that Mr. Felt was providing aid to the enemy in the wars of Watergate. The conversation was recorded in the Oval Office and later made public.

“We know what’s leaked, and we know who leaked it,” Nixon’s chief of staff, H. R. Haldeman, told the president on Oct. 19, 1972, four months after a team of washed-upCentral Intelligence Agency personnel hired by the White House was caught trying to wiretap the Democratic Party’s national offices at the Watergate complex.

“Somebody in the F.B.I.?” Nixon asked.

“Yes, sir,” Mr. Haldeman replied. Who? the president asked. “Mark Felt,” Mr. Haldeman said. “Now why the hell would he do that?” the president asked in a wounded tone.

No one, including Mr. Felt, ever answered that question in full. Mr. Felt later said he believed that the president had been misusing the F.B.I. for political advantage. He knew that Nixon wanted the Watergate affair to vanish. He knew that the White House had ordered the C.I.A. to tell the bureau, on grounds of national security, to stand down in its felony investigation of the June 1972 break-in. He saw that order as an effort to obstruct justice, and he rejected it. That resistance led indirectly to Nixon’s resignation.

Mr. Felt had expected to be named to succeed J. Edgar Hoover, who had run the bureau for 48 years and died in May 1972. The president instead chose a politically loyal Justice Department official, L. Patrick Gray III, who later followed orders from the White House to destroy documents in the case.

The choice infuriated Mr. Felt. He later wrote that the president “wanted a politician in J. Edgar Hoover’s position who would convert the bureau into an adjunct of the White House machine.”

Hoover had sworn off break-ins without warrants — “black bag jobs,” he called them — in 1966, after carrying them out at the F.B.I. for four decades. The Nixon White House hired its own operatives to steal information, plant eavesdropping equipment and hunt down the sources of leaks. The Watergate break-in took place six weeks after Hoover died.

While Watergate was seething, Mr. Felt authorized nine illegal break-ins at the homes of friends and relatives of members of the Weather Underground, a violent left-wing splinter group. The people he chose as targets had committed no crimes. The F.B.I. had no search warrants. He later said he ordered the break-ins because national security required it.

In a criminal trial, Mr. Felt was convicted in November 1980 of conspiring to violate the constitutional rights of Americans. Nixon, who had denounced him in private for leaking Watergate secrets, testified on his behalf. Called by the prosecution, he told the jury that presidents and by extension their officers had an inherent right to conduct illegal searches in the name of national security.

“As Deep Throat, Felt helped establish the principle that our highest government officials are subject to the Constitution and the laws of the land,” the prosecutor, John W. Nields, wrote in The Washington Post in 2005. “Yet when it came to the Weather Underground bag jobs, he seems not to have been aware that this same principle applied to him.”

Seven months after the conviction, President Ronald Reagan pardoned Mr. Felt. Then 67, Mr. Felt celebrated the decision as one of great symbolic value. “This is going to be the biggest shot in the arm for the intelligence community for a long time,” he said. After the pardon, Nixon sent him a congratulatory bottle of Champagne.

Mr. Felt then disappeared from public view for a quarter of a century, denying unequivocally, time and again, that he had been Deep Throat. It was a lie he told to serve what he believed to be a higher truth.

William Mark Felt was born in Twin Falls, Idaho, on Aug. 17, 1913. After graduating from the University of Idaho, he was drawn to public service in Washington and went to work for Senator James P. Pope, a Democrat.

In 1938, he married his college sweetheart, Audrey Robinson, in Washington. They were wed by the chaplain of the House of Representatives. She died in 1984. The couple had a daughter, Joan, and a son, Mark. They and four grandsons survive Mr. Felt.

Days before Pearl Harbor, after earning a law degree in night classes at George Washington University, Mr. Felt applied to the F.B.I. and joined it in January 1942. He spent most of World War II hunting German spies.

After stints in Seattle, New Orleans and Los Angeles, Hoover named him special agent in charge of the Salt Lake City and Kansas City offices in the late 1950s. Rising to high positions at the headquarters in the 1960s, he oversaw the training of F.B.I. agents and conducted internal investigations as chief of the inspection division.

In early 1970, while waiting in an anteroom of the West Wing of the White House, Mr. Felt chanced to meet a Navy lieutenant delivering classified messages to the National Security Council staff. The young man in dress blues was Bob Woodward. By his own description fiercely ambitious and in need of adult guidance, Mr. Woodward tried to wring career counseling from his elder. He left the White House with the number to Mr. Felt’s direct line at the F.B.I.

On July 1, 1971, Hoover promoted Mr. Felt to deputy associate director, the third in command at the headquarters, beneath Hoover’s right-hand man and longtime companion, Clyde A. Tolson. With both of his superiors in poor health, Mr. Felt increasingly took effective command of the daily work of the F.B.I. When Mr. Hoover died and Mr. Tolson retired, he saw his path to power cleared.

But Nixon denied him, and he seethed with frustrated ambition in the summer of 1972.

One evening that summer, a few weeks after the Watergate break-in, Mr. Woodward, then a neophyte newspaperman, knocked on Mr. Felt’s door in pursuit of the story. Mr. Felt decided to co-operate with him and set up an elaborate system of espionage techniques for clandestine meetings with Mr. Woodward.

If Mr. Woodward needed to talk, he would move a flowerpot planted with a red flag on the balcony of his apartment on P Street in Washington. If Mr. Felt had a message, Mr. Woodward’s home-delivered New York Times would arrive with an inked circle on Page 20. Mr. Woodward would leave his apartment by the back alley that night and take one taxi to a downtown hotel, then a second to an underground parking garage in the Rosslyn section of Arlington, Va.

Within weeks, Mr. Felt steered The Post to a story establishing that the Watergate break-in was part of “a massive campaign of political spying and sabotage” directed by the White House. For the next eight months, he did his best to keep the newspaper on the trail, largely by providing, on “deep background,” anonymous confirmation of facts reporters had gathered from others. The Post’s managing editor, Howard Simons, gave him his famous pseudonym, taken from the pornographic movie then in vogue.

By June 1973, Mr. Felt was forced out of the F.B.I. Soon he came under investigation by some of the same agents he had supervised, suspected of leaking information not to The Post but to The New York Times. He spent much of the mid-1970s testifying in secret to Congress about abuses of power at the F.B.I. Millions of Americans knew him only as a shadowy figure in the 1976 movie made from the Watergate saga, “All the President’s Men,” which made “Woodward and Bernstein” legends of American journalism. In the movie, Deep Throat (Hal Holbrook) gives Mr. Woodward (Robert Redford) probably the most famous bit of free advice in the history of investigative journalism. It was a three-word road map to the heart of the matter: “Follow the money.”

Mr. Felt never said it. It was part of the myth that surrounded Deep Throat.

William M Felt