FBI Informant Says Agents Missed Chance to Stop 9/11 Ringleader Mohammed Atta Undercover operative 'one million % positive' attacks could have been prevented.

Sept. 10, 2009 -- 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.

Watch the undercover agent's exclusive interview with Brian Ross tonight on World News with Charles Gibson and Nightline.

According to Assaad, Shukrujumah, whose father ran the mosque, invited the undercover FBI operative to meet him at his home, but the FBI told him to stay away. Instead, Assad says the agency assigned him to set up and sting what he calls wannabe terrorists, ending any hope of infiltrating the real al Qaeda terrorists.

Former national security official Richard Clarke, now an ABC News consultant, said the case is "yet another example of the way the system broke down prior to 9/11."

"If the system had worked," Clarke said, "we might have been able to identify these people before the attacks."

After 9/11

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, who wonder what the true value of the stings are. Since 9/11, the stings have largely targeted people that are more aspirational than operational.

"A lot of the cases after 9/11 were manufactured or enormously exaggerated and were announced with great trumpets by the attorney general and the FBI director so that we felt that they were doing something when, in fact, what they were doing was not helpful, not relevant, not needed," said Clarke.

While Assaad's undercover work has been called invaluable by the government in making the U.S. safer and has resulted in successful stings of alleged terrorist cells across the country, defense attorneys call him a master of entrapment against what they consider harmless targets.

After a 2006 sting, then-Attorney General Alberto Gonzalez praised Assaad – who was still unnamed at the time – for disrupting a group preparing a violent attack, resulting in the indictments of seven men on terror charges.

FBI undercover video from the case, obtained by ABC News, shows Assaad leading seven young men from Miami's inner city in a loyalty oath to Osama bin Laden, in which he persuaded them he had been sent by al Qaeda with tens of thousands of dollars to spend on them for weapons and training. The tape shows Assaad , who was working with the South Florida Joint Terrorism Task Force at the time, hugging his targets and welcoming them to al Qaeda.

Another video of a later meeting shows Assaad counting out $1000 for the leader of the Miami group.

After three trials, including two mistrials, five of the seven men were convicted.

Alleged Law Enforcement Entrapment

Defense lawyers including attorney Richard Houlihan, who represented Naudimar Herrera – one of the two men who were found not guilty, called the case a classic example of law enforcement entrapment.

"Without [Assaad's] performance, none of this would have happened," Houlihan told ABC News. "He's dealing with seven basically inner-city kids from Liberty City. Poor, uneducated, looking for money."

Herrera said he and the others played along with Assaad because of the financial incentives promised by him.

"He was like, "Oh your name here. You say your name here." It was more like, it was more like a movie script," Herrera told ABC News. He said that anyone "blind about greed" would be "vulnerable to [Assaad's] intelligence."

Assaad said some tactics – like suggesting targets, as he did in this case with FBI offices in Miami – are necessary in undercover stings.

"Sometimes you have to see…what he's willing to do….what's he's capable of doing," says Assaad of suspected terrorists.

"When you are working undercover," Assaad says, "your job is to lie."

Megan Chuchmach contributed to this report.

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