Was There a 20th Hijacker for Sept. 11?

"We tend to fixate on the missing 20th hijacker as the only missing piece of the 9/11 puzzle," said Bruce Hoffman, a terrorism expert who heads the Washington office of Rand Corp., a public policy think tank. "But that precludes that other simultaneous attacks may have been planned or that some other assault teams just couldn't be deployed. . . . There are still many other missing pieces."

Immediately after Sept. 11, Moussaoui emerged as the leading candidate for 20th hijacker, in part because of a suggestion from Cheney and because he was arrested in Minnesota while seeking flight training on jumbo jets.

Moussaoui encouraged the speculation by calling himself the "20th hijacker" in handwritten motions filed from jail, usually in a mocking tone, and in his written agreement to plead guilty. During his sentencing trial, Moussaoui also claimed -- and later denied -- that he was supposed to fly a fifth hijacked plane into the White House.

Prosecutors repeatedly pointed out that they never called Moussaoui the 20th hijacker in any court proceeding. "The '20th hijacker' theory appears to be a creation of the media coverage and the isolated statements of certain government officials in the immediate aftermath of the Sept. 11 attacks," they wrote in 2003.

Juliette N. Kayyem, a counterterrorism expert and lecturer at Harvard University's Kennedy School of Government, said the Nashmi and bin Laden tapes suggest that al-Qaeda is fixated on past events, either for propaganda or to relive successes.

"Their obsession with the 20th hijacker is about as relevant as our obsession with Zacarias Moussaoui," Kayyem said. "It has very little relevance to what is going on today. . . . My fear is not that there is a 20th hijacker; my fear is that there are 2,000 hijackers now."

Staff writers Dafna Linzer and Jerry Markon and researcher Julie Tate contributed to this report.