On the eve of the eight year anniversary of the 9/11 attacks, an FBI informant who infiltrated alleged terrorist cells in the U.S. tells ABC News the FBI missed a chance to stop the al Qaeda plot because they focused more on undercover stings than on the man who would later become known as 9/11 ringleader Mohammed Atta.

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."

The FBI did not specify which claims or conclusions it referred to.

Asaad said he told ABC News the truth and stands by his story.

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