Look at the photo of W. Mark Felt as a 45-year-old FBI agent in 1958, and you begin to grasp something of the terrible ambivalence he endured over his role as Deep Throat. Felt appears poised for action, knees bent. In one hand, a drawn gun points at the camera; the other hand is raised slightly, as if to tell a bad guy, don't mess with me, fella. His tie is fashionable, even edgy; his fedora is tilted just so, a handkerchief peeks from his breast pocket. You can imagine him practicing the pose in front of the bedroom mirror before heading out to meet the newsmen from the local paper.

Even the name, Mark Felt, fits -- matter-of-fact, monosyllabic, the William condensed to a terse, Hooveresque initial. He seems, in short, the perfect G-man. And G-men -- not the G-men of legend, anyway -- don't leak, don't skulk around parking garages dispensing Delphic tips.

It seems almost obvious, in retrospect, that Deep Throat came from the FBI. Anyone else would have emerged from anonymity decades ago to revel in the glory of being the world's most famous unnamed source. We'll never know, not for certain, what Felt thought of himself as Deep Throat, what it meant for him to carry that secret around for so many years and to deny his role so categorically. Secrets are corrosive, the self-help gurus insist; they must be purged. Yet the more you read about Felt, the more you understand why, for him, keeping this secret buried may have been the better course.

"It was not I, and it is not I," Felt said in 1974 in denying that he was Deep Throat, and there is a sense in which that may have been not so much an outright lie as a deeper truth for Felt: In fact, he was not behaving like himself, or at least like the person he wanted to be. When Slate's Timothy Noah tracked him down in 1999, Felt said it "would be terrible" if he had been Deep Throat: "This would completely undermine the reputation that you might have as a loyal, logical employee of the FBI. It just wouldn't fit at all."

To comprehend how thoroughly Felt believed that it wouldn't fit for him to be both Mark Felt and Deep Throat, consider how insistently he kept his secret hidden from his own family. The more you read of Vanity Fair's account of the outing, the sorrier you feel for a failing old man prodded and even tricked by his relatives into telling all -- to get "closure," as his daughter put it, perhaps finally to profit from what the family, if not Felt, viewed as his heroism.

Felt may have had lofty goals in helping Woodward. But it is the rare source whose leak is solely a matter of altruism, and Felt, I suspect, had -- and at some level knew he had -- baser motives as well. The most obvious, and most unattractive, is the fact that he had just been spurned for the top job at the FBI. The astringency of Felt's account in his autobiography masks the depth of his hurt: "My own record was good and I allowed myself to think I had an excellent chance," he wrote. There were other possible motives at work, too: the urge to protect Hoover's FBI from White House interference, the thrill of the spymaster-turned-bureaucrat at the chance for one last operation.

Richard Nixon would have understood Felt's reluctance to be unmasked, even if he guessed wrong about him. In one of the grand pronouncements he was given to making on the White House tapes, Nixon weighed, and dismissed, the possibility that Felt was the leaker. "The informer is not wanted in our society," he told White House counsel John Dean. "Either way, that's the one thing people do sort of line up against." Nixon assumed that Felt, if he were the secret source, would eventually be revealed and would pay the price. "Everybody would treat him like a pariah," Nixon said. "He's in a very dangerous situation."

Just a few years after his clandestine role in toppling the president, Felt found himself on trial for another clandestine operation. He was charged with authorizing "black bag" jobs, break-ins at the homes of relatives of members of the Weather Underground. In this case, Felt and his co-defendant were enveloped with FBI support, agents thronging the courtroom in a show of solidarity. What would they have thought, Felt must have worried, if they knew his true, his alter-identity? Through all the years he steadfastly denied being Deep Throat, this was the audience whose reaction seemed to most concern him.

Flash forward from the crisp image of Felt as FBI man to the photographs of the frail 91-year-old of today, leaning on his walker, his plaid shirt untucked. I am glad, I suppose, to finally know the secret of Deep Throat. I am less confident that Mark Felt wanted me to know.

marcusr@washpost.com