On Tuesday, Oct. 10, 1972, Washington awoke to this startling news:

“FBI agents have established that the Watergate bugging incident stemmed from a massive campaign of political spying and sabotage conducted on behalf of President Nixon’s reelection and directed by officials of the White House and the Committee for the Re-election of the President,” The Washington Post reported.

Bob Woodward and Carl Bernstein wrote that story, but it would not have been possible, they would later tell the world, without the clandestine help of the most famous unnamed source in history, W. Mark Felt -- the man they called “Deep Throat.”

He was the associate director of the FBI, and he guided the reporters, all along insisting that Watergate was not an isolated incident but part of a wider conspiracy designed to destroy Democratic presidential candidates who might run against Nixon.

Instead, the Watergate scandal subsequently destroyed the Nixon presidency. Facing impeachment, Nixon resigned the presidency in disgrace less than two years after that day in October when, one of his key lawyers would later say, it was apparent that “in a general way and with the help of Deep Throat, (Woodward and Bernstein) figured out Watergate.” Felt died Thursday at his home in Santa Rosa, Calif. He was 95.

As the second-ranking official in the FBI, Felt possessed authoritative knowledge of what came to be known as the Watergate conspiracy. Felt had personally witnessed the Nixon administration’s attempt to subvert the bureau’s investigation into the complex of crimes and coverups that began unraveling with a 1972 burglary of the Democratic National Committee’s headquarters at the Watergate complex in Northwest Washington.

“He helped stiffen the spine of The Washington Post, which went out on a limb more than any other publication did, and broke stories that other outlets shunned,” said Mark Feldstein, a George Washington University faculty member who is writing a book on Nixon and the media. “Had Woodward not run with it, Felt had a network of connections in Washington, (other reporters) he was talking to . . . but no one knew of his connection to Woodward.”

Felt, a dashing figure with a full head of silver hair and a reputation as a taskmaster, adamantly denied over the years that he was Deep Throat, even though Nixon suspected him almost from the start. Nixon ordered interim FBI director L. Patrick Gray to fire Felt five times. But Gray, convinced by Felt’s denials, never did.

A master of bureaucratic infighting and misdirection, Felt seized upon a Post story that had not used him as a source. In a bold stroke, he denounced it in an internal memo and ordered an investigation into the leak. “Expedite,” he commanded. The next day, in a notation on another memo that passed across his desk, he pointed to a prosecutor as the source of the leak.

“I was impressed. My guy knew his stuff,” Woodward wrote in “Secret Man: The Story of Watergate’s Deep Throat” (2006). “The memo was an effective cover for him, the very best counterintelligence tradecraft. Not only had he initiated the leak inquiry, but Felt appeared to have discovered the leaker.”

Felt had insisted on remaining completely anonymous, or on “deep background.” A Post editor dubbed him “Deep Throat,” a bit of wordplay based on the title of a pornographic movie of the time. The source’s existence, but not his identity, became known in Woodward and Bernstein’s 1974 book, “All the President’s Men,” and in the subsequent movie version, in which actor Hal Holbrook played the charismatic but shadowy source.

It wasn’t until May 30, 2005, that Felt’s family revealed his identity in an article for Vanity Fair magazine. The story, written by San Francisco lawyer John D. O’Connor, did not make clear why Felt, who was suffering from dementia, admitted his role after more than 30 years. Woodward confirmed the revelation, and the secret was finally out.The Watergate controversy erupted all over again, with former Nixon aides denigrating Felt. “I think Deep Throat is a snake,” former Nixon speechwriter Patrick Buchanan told CNN in 2005.

But others said Felt did what had to be done.

“I do think Felt was justified in leaking because Nixon had corrupted the standard channels,” said Ken Hughes, a research fellow who has studied the Nixon tapes at the University of Virginia’s Miller Center of Public Affairs.

Few could imagine such a straight-arrow career employee, known for enforcing the FBI’s strict rules of behavior and demeanor, playing such a dangerous game. A loyalist to longtime FBI director J. Edgar Hoover, Felt supported Hoover’s eavesdropping on the Rev. Martin Luther King during the Kennedy administration. He opposed Gray’s decisions to hire women as FBI agents, loosen the dress code and ease weight restrictions for FBI agents.

Although Deep Throat was a hero to the counterculture, civil rights advocates and Nixon’s opponents, Felt was no friend to the political left.

In 1980, he was convicted of approving illegal “black bag” break-ins of the homes of people who the FBI believed were connected to the radicals of the violent Weather Underground movement. Felt was later pardoned by President Ronald Reagan.

No one knows exactly what prompted Felt to leak the information from the Watergate probe to the press. He was passed over for the post of FBI director after Hoover’s 1972 death, a crushing career disappointment.

But by the time he told O’Connor “I’m the guy they used to call Deep Throat,” he was enfeebled by a stroke and his memory of the era had almost vanished because of Alzheimer’s disease.

In his 2006 book with O’Connor, “A G-Man’s Life,” Felt expressed his anger at White House officials who attempted to interfere with the FBI investigation.

“It’s impossible to exaggerate how high the stakes were in Watergate,” he and his co-author wrote. “We faced no simple burglary, but an assault on government institutions, an attack on the FBI’s integrity, and unrelenting pressure to unravel one of the greatest political scandals in our nation’s history.

“From the start, it was clear that senior administration officials were up to their necks in this mess and would stop at nothing to sabotage our investigation. White House staffers, high and low, were either evasive or obstructive. They drew the Justice Department and the CIA into their cover-up. They used the acting director of the FBI, a political appointee, to inform them of the information we dug up and attempt to limit our inquiries. . . .

“I really can’t describe adequately how bad it was,” the book went on. “As investigators trying to bring the truth to light, we could not rely on Justice Department prosecutors or even federal grand juries to bring indictments. What we needed was a ‘Lone Ranger’ who could bypass the administration’s hand-picked FBI director and Justice Department leadership and derail the White House cover-up.”

Felt, who saw all the FBI investigative paperwork on Watergate, was acquainted with Woodward from a chance meeting at the White House in 1970, when Woodward was still in the Navy. After Woodward became a reporter, Felt helped him on a story about the attempted assassination in May 1972 of George C. Wallace Jr., the segregationist Alabama governor then running for president.

Days after the June 17, 1972, break-in at the Watergate, Felt told Woodward that The Post could safely make a connection between the burglars and a former CIA agent working at the White House, E. Howard Hunt.

Months later, Felt again provided key context and reassurance, telling Woodward that a story tying Nixon’s campaign committee to the break-in could be “much stronger” than the first draft and still be on solid ground.

One of the most important encounters between Woodward and his source came Oct. 8, 1972. In the early hours in a deserted parking garage in Rosslyn, Felt laid out a much broader view of the scandal than Woodward and Bernstein had yet imagined.

“On evenings such as these, Deep Throat had talked about how politics had infiltrated every corner of government -- a strong-arm takeover of the agencies by the Nixon White House. . . . He had once called it the ‘switchblade mentality’ -- and had referred to the willingness of the president’s men to fight dirty and for keeps,” Woodward and Bernstein wrote in “All the President’s Men.” “The Nixon White House worried him. ‘They are underhanded and unknowable,’ he had said numerous times.”

Felt urged Woodward to follow the case to the top: to Nixon’s former attorney general, John N. Mitchell; to Nixon’s inner brace of aides, H.R. “Bob” Haldeman and John D. Ehrlichman; and even to Nixon himself.

“Only the president and Mitchell know” everything, he hinted.

It took many newspaper stories (not all of them first reported by The Post), a House and Senate investigation, the revelation of a secret tape recording system in the Oval Office, the firing of a special prosecutor, the opening of articles of impeachment and the discovery of a “smoking gun” tape recording before Nixon resigned Aug. 9, 1974.

The Post won journalism’s highest honor, the Pulitzer Prize for public service, in 1973 for its investigation of the Watergate case.

That same year, Felt was passed over for the job of FBI director a second time, and he retired from the bureau that summer.

But in 1978, he was drawn back into the public view when he and another top FBI official, Edward Miller, were indicted for nine break-ins in New York and New Jersey that had happened in 1972 and 1973.

Felt said he believed he was acting with the approval of Hoover when he authorized the break-ins of people who the FBI believed were connected to the Weather Underground. When he was arraigned, several hundred FBI agents greeted him at the courthouse in a show of solidarity.

It was during that period that Felt came closest to disclosing his secret. Under questioning by grand jurors, he cavalierly mentioned that he was often suspected of being Deep Throat. A grand juror immediately asked him if he was. Felt, according to assistant attorney general Stanley Pottinger, turned pale and denied it. According to Woodward’s book, Pottinger went off the record, reminded Felt that he was under oath and offered to withdraw the “irrelevant” question if Felt preferred. Withdraw it, Felt snapped.

Few others came that close. After Felt’s wife, Audrey, committed suicide in 1984, Felt told a close friend, Yvette LaGarde, of his secret identity, and she told her son. Over the years, speculation about the identity of the secret source became a journalistic parlor game; Post columnist Richard Cohen and author Ronald Kessler both suggested Felt as the source.

On the day of his conviction in 1980, Felt told reporters, “I spent my entire adult life working for the government, and I always tried to do what I thought was right and what was in the best interest of this country and what would protect the safety of this country.”

Five months later, Reagan pardoned Felt and Miller.

William Mark Felt Sr. was born Aug. 17, 1913, in Twin Falls, Idaho, the son of a general contractor and a housewife. He worked his way through the University of Idaho, waiting tables and stoking furnaces, and graduated in 1935.

He moved to Washington to work for two Idaho Democrats, Sen. James P. Pope and then Sen. David Worth Clark, while attending night law school at George Washington University. He graduated in 1940.

After law school, he worked briefly at the Federal Trade Commission, where he was assigned to ask consumers about their impressions of the Red Cross brand of toilet paper. Unhappy with the job, he joined the FBI in 1942.

Assigned to counterintelligence work, he thrived. In his 1979 book “The FBI Pyramid,” Felt said he learned techniques and the uses of misinformation that allowed him to unmask a German spy on U.S. soil just before World War II.

After the war, he chose to go to the FBI’s Seattle office, then Houston, San Antonio, Washington, New Orleans, Los Angeles and Salt Lake City, where he was named special agent in charge in 1956. He led the FBI field office in Kansas City, a town that was a hotbed of political corruption, before finally moving back to Washington in 1962 -- 17 moves by the time he retired.

In 1964, Felt began a six-year stint as the FBI’s chief inspector, making sure that all agents and field offices toed the line on regulations, an outfit known internally as the “goon squad.” During that time, he also served as FBI liaison and technical adviser to MGM Studios for “The FBI” television series.

Promoted to deputy associate director in 1971, he was, by all accounts, loyal to Hoover. He was also suspicious of the Nixon White House effort to bring the FBI under its control. He resisted a directive from the White House in 1971 to begin massive wiretaps to find the source of leaks about the administration’s national security strategy. But he won the administration’s confidence when he quietly closed a Hoover-ordered investigation into “a ring of homosexualists at the highest levels,” an allegation that proved unfounded.

In early 1972, the administration was embarrassed by a memo from ITT lobbyist Dita Beard that said if her employer contributed to Nixon’s campaign fund, the Justice Department would drop its antitrust investigation. Hoping to prove the memo was a forgery, the White House sought the FBI’s cooperation. But Felt reported that the FBI laboratory could not make a definitive finding. White House special counsel Charles W. Colson pressured Felt to change the FBI’s summary of its investigation, but Felt would not budge.

“We really don’t know to this day everything that Mark Felt did or didn’t do in Watergate and his motives,” said GWU’s Feldstein. “All we really know is what Bob Woodward said and what Felt wrote in that first memoir. . . . Fundamentally, he was a loyal aide-de-camp to J. Edgar Hoover, who carried out Hoover’s will. Watergate was really a tiny percentage of what he did. He has a lot to answer for up there at the pearly gates.”

After Hoover’s death May 2, 1972, Nixon appointed Gray as acting director of the agency. Felt was infuriated by Gray’s capitulation to the administration’s demands, including turning over FBI investigative files to the White House staff. But he succeeded in persuading Gray to resist Nixon’s attempt to get the FBI off the case of the Watergate burglary.

While Gray worked with the White House and spent part of each week visiting most of the FBI’s bureaus across the country, Felt was in operational charge of the agency. But after a disastrous confirmation hearing, Gray resigned, and Nixon refused to promote Felt, instead appointing William D. Ruckelshaus to the top FBI spot.

Ruckelshaus soon accused Felt of leaking information about illegal wiretaps -- not to The Washington Post, but to the New York Times. Felt angrily denied the charge, then immediately retired in 1973. Even in retirement, he stayed in touch with sources and reporters and tipped off Woodward one last time. The secret White House tape recordings that were rumored to exonerate Nixon contained “one or more . . . deliberate erasures,” he said.

Felt moved to Santa Rosa from Alexandria in 1989. He suffered a stroke in 1999 and a second one in 2001. He was under hospice care at home when he died.

He met the other half of the Woodward and Bernstein duo just last month.

His son, Mark, became an Air Force pilot and flew then-Vice President George H.W. Bush in Air Force Two. His daughter, after living a countercultural life in California, became a teacher and lives in Santa Rosa. Survivors also include several grandchildren.

Because of questions about his memory by 2005, it is unclear whether Felt or co-author O’Connor wrote in his last book: “People will debate for a long time whether I did the right thing by helping Woodward. The bottom line is that we did get the whole truth out, and isn’t that what the FBI is supposed to do?”

Associate Editor Bob Woodward and staff writers Anita Kumar, Martin Weil and Clarence Williams contributed to this report.