BROOKE GLADSTONE: Since All the President's Men hit bookshelves in 1974, Deep Throat code breakers have been using the clues that Woodward and Bernstein peppered throughout their book to concoct their theories on Deep Throat's identity. William Gaines and his students at the University of Illinois used the available clues to try and crack the mystery. We spoke to Gaines in 2003.

WILLIAM GAINES: I saw a list of the ten greatest mysteries of the 20th Century, and Deep Throat was number five. Amelia Earhart was number one. And it struck me at the time that nobody knows what happened to Amelia Earhart, but somebody knows who Deep Throat is and they're not telling us. So I guess that became a challenge, and also added to that challenge was a statement that Ben Bradlee, you know, the editor of Woodward and Bernstein at the time, made recently that somebody could probably take all the information known about Deep Throat and put it in some huge computer, and they would be able to figure out who Deep Throat was.

BROOKE GLADSTONE: Gaines and his students figured out that Deep Throat had to be - Fred Fielding. They were, apparently, wrong. But who can blame them? It turns out, the clues that were supposed to lead us there were misleading us all along. Tim Noah, a longtime Deep Throat sleuth who writes the Chatterbox column for Slate, complained this week, and he joins me now. Tim, welcome back to OTM.

TIM NOAH: Thank you, Brooke.

BROOKE GLADSTONE: Let's first give credit where credit is due. Despite the misdirections, you and others have been saying Mark Felt for a while. James Mann, Nora Ephron, Richard Nixon, Washingtonian Magazine. How did you first land on Mark Felt for sure?

TIM NOAH: I should confess, before I proceed, that in recent years, I had started to wonder if it was Fred Fielding. But yes, the person I've stuck with most was Mark Felt. And the evidence seems pretty compelling. It was laid out in a magazine article by James Mann in 1992 that Deep Throat had to be someone at the FBI, and one of his reasons for believing this was that, during the period immediately before and during the Watergate stories, Mann worked at the Washington Post and sat next to one Bob Woodward, who was always shooting his mouth off about his hot FBI source.

BROOKE GLADSTONE: But what are the clues that could have sent you off the trail to identifying Mark Felt?

TIM NOAH: Well, there were principally two. One is that Deep Throat is described as a smoker. But Felt was not a smoker. He had quit smoking in the 1940s. So one of two things has to be true. Either Woodward and Bernstein included the smoking detail as misdirection, or Felt has a second secret, which is that he's been sneaking cigarettes.

BROOKE GLADSTONE: And that certainly sent William Gaines off on the wrong track.

TIM NOAH: That's right. Well, it's like the data processing line - garbage in, garbage out.

BROOKE GLADSTONE: Now, you said there were two bits of garbage that through you off - first the smoking; the second, where Deep Throat worked, or where Woodward said he didn't work. You believed him, when he said in a 1979 Playboy interview that Deep Throat didn't work in an intelligence agency.

TIM NOAH: That's right. There was a Playboy interview, and Anthony Lukas, a very accomplished late journalist, asked Woodward if Deep Throat was a member of the intelligence community, and Woodward, in so many words, said no. Now, he was a member of the intelligence community. I've gotten some letters from people saying that the FBI is not an intelligence agency. It's a crime-fighting agency. And I had talked myself into that for a while, but then I thought about the historical context, and this was a period immediately after a spell when the FBI was spying on seemingly everybody and people thought of it at the time, as these revelations were coming out in the late '70s, in the wake of J. Edgar Hoover's death, clearly the FBI was part of the intelligence community. And Woodward writes about it as such in the long narrative that was in the Washington Post on Thursday.

BROOKE GLADSTONE: Well, Tim, is a little misdirection to protect a source so very bad?

TIM NOAH: I think journalists have an obligation, always, to be truthful, even about the most trivial things. Journalists swarm over government officials when they are untruthful, even about trivial things, and it would be a double standard to do otherwise. And, and besides, as a reader, I want to read something that's truthful. I want to know, when I'm reading an article in the Washington Post that every word in it is true, to the best of the ability of the reporter who wrote it.

BROOKE GLADSTONE: We, as journalists, always say that reporters should never reveal their sources, cause it will discourage other potential leakers and whistleblowers from coming forth. Well, here, Woodward took that extra step of obscuring his identity by saying he didn't work for intelligence, that he was a smoker, wouldn't that build the confidence of would be leakers, knowing that their identity would be so very obscured?

TIM NOAH: When leakers ask for that kind of protection, they're asking for too much. They can ask that you disguise his identity through misdirection, but I don't think journalists should grant that wish.

BROOKE GLADSTONE: But would you have believed Hal Holbrook as much if he wasn't puffing in that garage?

TIM NOAH: You know, I got an interesting email yesterday from someone who said, you know, Hal Holbrook - all wrong for Mark Felt. You want Christopher Walken. He's got the hair. [LAUGHTER]

BROOKE GLADSTONE: Thanks very much, Tim.

TIM NOAH: Thanks, Brooke.

BROOKE GLADSTONE: Tim Noah writes the Chatterbox column for Slate. [CLIP FROM ALL THE PRESIDENT'S MEN PLAYS]

DEEP THROAT/HAL HOLBROOK: It involves the entire U.S. intelligence community - FBI, CIA and Justice. Get out your notebook. There's more. Your lives are in danger.

BOB GARFIELD: Coming up, how to program for a continent and how not to.

BROOKE GLADSTONE: This is On the Media, from NPR.