Nothing in the saga of the fall of Richard Nixon has riveted Americans more than the identity of "Deep Throat," whose parking-garage advice led the way to the truth for a pair of young reporters.

On the eve of the publication of "Unmasking Deep Throat," a book by former Nixon aide John Dean that Dean said has reduced to "about a thimbleful" the names of people who might be Bob Woodward and Carl Bernstein's famed anonymous source, the writers held to their 30-year silence Sunday.

Woodward, Bernstein, Dean and other Watergate figures reminisced on Sunday's television talk shows about the 1970s scandal that began with the pre-dawn June 17, 1972, break-in of an office in Washington's elegant Watergate building and ended more than two years later with the resignation of the 37th president.

Woodward, asked about a project by university students in Illinois that eliminated all but seven White House aides as Deep Throat possibilities, and the students' unanimous surmise that Deep Throat was commentator and former Nixon aide Pat Buchanan, Woodward said: "You're going to get a kind of deep silence from us on this subject."

"We have talked about it, and we had eliminated some people, but lots of people have died, and the list is narrowed. And for us it's really a principle. It's about keeping our word for 30 years, because the process of reporting involves having those confidential sources who will say, `This is what's really going on."'

As he has before, Woodward said on NBC's "Meet the Press" that their source's identity would be revealed only after the source dies or releases the reporters from their confidentiality pledge.

Dean's 158-page e-book included the names of five people he said may have been Deep Throat, including Buchanan and Ron Ziegler, Nixon's press secretary. The $8 book is being sold on the Web site of Salon.com.

The list also included Steve Bull, an assistant to Nixon appointment secretary Dwight Chapin; Raymond Price, a special assistant to Nixon; and Jerry Warren, Ziegler's assistant.

Dean writes in the preface that trying to nail down Deep Throat's identity remains a "tricky" and "unpredictable" task.

Charles Colson, a Nixon aide whose role in another break-in orchestrated by the White House cost him a felony conviction and seven months in prison, said lessons of Watergate have not been learned.

"What we should learn is not to put our trust and faith in men who are corruptible," said Colson, who as a presidential aide ventured that "I would run over my grandmother to re-elect Nixon."

Colson noted that Congress has just overturned post-Watergate restrictions on political fund raising. "Those reforms didn't work," Colson said on Fox News Sunday, because "human nature doesn't change."

Dean's "cancer on the presidency" warning to Nixon was the first sign of slippage in the presidential advisers' code of silence. He said on CBS' Face the Nation that when he broke ranks, "I realized my days were numbered, because I could no longer serve as the desk officer of the cover-up and be giving him that kind of advice."

"I don't think initially he wanted to make me a scapegoat. He wanted to sort of protect himself through me," Dean said. It was only later, he said, that Nixon decided that "everybody was expendable but himself."

Branded a traitor by Nixon and fired, Dean gave damning testimony before the Senate Watergate Committee.

Bernstein and Woodward, whose dogged intrusions into the Nixon presidency's darkened closets won The Washington Post a Pulitzer Prize, spoke on NBC of the lessons for journalists that grew out of their work. The lessons are largely ignored after three decades, Bernstein said.

"The lessons have to do with being careful, with using multiple sources, to putting information into context, to not being swayed by gossip, by sensationalism, by manufactured controversy. All of which I think have come to dominate our journalistic agenda much more in the past 30 years," said Bernstein, who has left daily journalism and now writes books.