Mark Felt: FBI official revealed as 'Deep Throat' informant 30 years after Watergate

For three decades it remained the last riddle of Watergate. Just who was "Deep Throat", the federal government official who from the shadows of an underground car park guided Bob Woodward and Carl Bernstein of The Washington Post in the treacherous early months of the investigation that toppled a President?

Only four people were in on the secret: Woodward and Bernstein, the Post's then executive editor, Ben Bradlee, and of course "Deep Throat" himself – unless, as the odd disbeliever would maintain, he was a composite created after the event in order to boost sales of the reporters' 1974 best-seller All the President's Men.

Such cynicism was misplaced. On 31 May 2005, almost 33 years after the fateful break-in of Richard Nixon's plumbers at the Democratic Party headquarters in Washington, the mystery was finally lifted. The most famous unidentified source in the history of American journalism was revealed as a single individual, a frail old gentleman of 91 living in distant California. His name was Mark Felt.

The great scoop, obtained by Vanity Fair magazine, was not entirely unexpected. The suspects for "Deep Throat" had been an exotic bunch, ranging from Al Haig, former Secretary of State and Nixon's chief of staff, to Henry Kissinger, the Nixon speechwriter Pat Buchanan, and even the 1970s White House press aide and now ABC news journalist Diane Sawyer.

But for serious students of Watergate, Felt – deputy director of the FBI at the time of the scandal – was always a leading candidate. Few considered it likely that any of Nixon's praetorian guard of senior aides would betray the President. Felt, as a top official of the agency investigating the break-in of 17 June 1972, had all the access but none of the loyalty; indeed, quite the contrary.

Not only was he deeply resentful at having been passed over by Nixon for the top job at the FBI after the death of J. Edgar Hoover a few weeks earlier. From the outset of Watergate he was convinced that the White House was bent on a cover-up. In a spy novel, Felt would have been a perfect mole. In the context of Watergate, he was a natural whistle-blower.

Thus his involvement began. The contact man was Woodward. The two had first met in a White House ante-room in 1970. Felt a senior FBI man waiting for an appointment, while Woodward was a young military intern – but already possessing a keen instinct of the value of well-placed contacts. Even before he moved to the Post, Woodward was at pains to nurture their acquaintanceship.

Crucially, on that first encounter, he extracted Felt's direct office phone number. When Woodward, a rookie local reporter, was assigned to Watergate, all the pieces came together: the colossal story, the gifted young journalist, and the source so perfectly placed it hardly seemed true.

Felt's counter-espionage work against suspected Nazis in the United States during the Second World War had given him a taste for the cloak and dagger, and the stakes in Watergate were scarcely less. His contacts with Woodward were the stuff of John le Carré. Direct phone conversations, even daylight meetings, were out. If Woodward wanted to talk, he would signal his desire by moving a flowerpot on his apartment balcony.

If Felt needed a meeting, he would send a message in Woodward's daily New York Times (how Felt got to the newspaper in question, the latter never learned). They met at dead of night in a car park in Rosslyn, Virginia, across the Potomac river from Georgetown. As instructed, Woodward never travelled directly. Often, he doubled back on his tracks to throw off any tail.

All the President's Men gave few specific clues to the identity of "Deep Throat". He was fond of Bourbon and Scotch, and fascinated by – although properly sceptical of – gossip and rumour. The real-life Felt, debonair and dashing, often to be seen in classic snap-brim trilby, fitted the bill. For Woodward and Bernstein, he was priceless; not because of new information that he provided, but for his ability to point them in the right direction, and help them avoid mistakes in a fiendishly complicated story in which the reporters' necks (not to mention the reputation of The Washington Post) were always on the line.

Felt left the FBI in mid-1973, more than a year before Nixon finally resigned, but he had long since done his job. By then, other newspapers were making the running along with the Post, and what had started as a criminal case had moved to Capitol Hill and ultimately the Supreme Court.

"Deep Throat", it should be said, was not decisive in Nixon's downfall. Equally important parts were played by judges and special prosecutors, by cabinet officers like the Attorney General Elliot Richardson, by the sleuthing of Woodward and Bernstein – not to mention Frank Willis, the security man that June night at the Watergate building without whose alertness the break-in would never have been detected.

After the event, Felt always strenuously denied his role, first in his 1979 memoir The FBI Pyramid From the Inside, and then in 1992, after an Atlantic Monthly article in which the journalist James Mann persuasively argued that Felt was the most likely suspect. But not until May 2005 and the Vanity Fair article, written by a lawyer and family friend to whom Felt had told his story, did confirmation come.

His biggest dislike was his nickname, bestowed upon him by Howard Simon, the Post's managing editor, after the porn movie that was then the sensation of the day. But after the 1976 film version of All the President's Men, starring Robert Redford, Dustin Hoffman and Jason Robards as Woodward, Bernstein and Bradlee respectively (and featuring Hal Holbrook as a rasping "Deep Throat"), there was nothing Felt could do about it.

His last brush with notoriety came in 1980 when he and other senior FBI officials were convicted of ordering illegal break-ins without search warrants during a 1972/73 investigation of the radical Weather Underground group. But he was pardoned by the incoming President, Ronald Reagan, the following year.

Curiously, Felt's unmasking came amid a crisis of credibility for mainstream US news organisations, amid a fierce debate over the use of unnamed sources – the infamous "senior administration officials" who had provided wrong information about Saddam Hussein's non-existent WMD before the 2003 invasion of Iraq.

How different from three decades earlier, when the Post's Watergate heroics lifted journalism's prestige to heights unattained before or since. For a while after the Iraq débâcle, many papers even considered dispensing with unattributed information entirely. But without the likes of "Deep Throat", Watergate would never have become journalism's finest hour, and one of the stories of the century could never have been told.

William Mark Felt, intelligence officer: born Twin Falls, Idaho 17 August 1913; staff, FBI 1942-73; married Audrey Robinson (one son, one daughter); died Santa Rosa, California 18 December 2008.