W. Mark Felt, a former FBI Official, told Vanity Fair that he was the anonymous source known as Deep Throat. The report counter's both Felt's 1999 denial to the Hartford Courant saying that he would have done better had he been Deep Throat.

In 2003 a University of Illinois journalism class published the results of a four-year investigation in which they name Fred Fielding, deputy counsel to former President Richard Nixon as the most likely source for the information.

James Mann argued the idea that Deep Throat was from the FBI in his May 1992 article for the Atlantic Monthly. In the article he quotes from Felt's 1979 memoir about finding a replacement for J. Edgar Hoover:

It did not cross my mind that the President would appoint an outsider to replace Hoover. Had I known this, I would not have been hopeful about the future. There were many trained executives in the FBI who could have effectively handled the job of Director. My own record was good and I allowed myself to think I had an excellent chance.

Mann goes on to suggest that the fierce independence of the FBI was threatened by Nixon's appointment of an outsider to head the agency that would ultimately investigate the June 17th break-in at the Watergate Hotel.

Update: The story has been confirmed by Bob Woodward and Ben Bradlee of the Washington Post. James W. Crawley has a interesting story My Uncle Deep Throat.