Elie Assaad

In an exclusive interview to be broadcast tonight on ABC World News with Charles Gibson and Nightline, former undercover operative Elie Assaad says he spotted and became suspicious of Atta in early 2001, when he was sent by the FBI to infiltrate a small mosque outside Miami. Atta was there with Adnan Shukrujuman, an al Qaeda fugitive who now has a $5 million U.S. reward on his head.

"There was something wrong with these guys," Assaad, a 36-year-old Catholic native of Lebanon who pretended to be an Islamic extremist, says.

The FBI initially declined to comment but released a statement following the ABC News report, saying: "The 9/11 investigation, the most extensive ever conducted by the FBI, has been reviewed in its totality by the 9/11 Commission, Congress and others. The claims made in the news report and the factual conclusions contained in the story are not supported by the evidence."

Assaad, who posed as "Mohammed" – a personal representative of Osama bin Laden, says he's a "million percent positive" the 9/11 attacks could have been stopped if the FBI had gone after Atta and Shukrujumah. But because Atta and his men were suspicious of the FBI undercover operative, and secretive, Assaad says his FBI agent handlers sent him after the easier target – two wannabe terrorists whose cases were easy to crack and who were both eventually convicted and sent to prison.

"I was right, I was a hundred percent right," Assaad says of his suspicions. He says that when he learned that Atta was one of the 9/11 hijackers, when the FBI asked if he could identify any of the attackers, he was "very upset, angry" and cried.

"I curse on everybody," Assaad says. "I destroyed half of my furniture. Uh, I went crazy."

The FBI's focus on stings, which Assaad has worked in at least 10 states and overseas since becoming an operative in 1996, are being questioned by many counter-terrorism authorities. j