The informant, a Sudan-born driver and confidante to "Blind Sheik" Omar Abdel-Rahman, the alleged mastermind the first bombing attempt on World Trade Center, was the lone asset providing first-person information about al-Qaida in the mid-1990s, NBC News said.
The Egyptian-born Rahman had been an FBI target since at least Nov. 5, 1990, when one of his followers shot and killed radical Rabbi Meir Kahane in a Manhattan hotel. In the aftermath, information found in Nosair's apartment suggested a larger conspiracy. When the sheik moved some of his operations from New York to Los Angeles, the person who eventually became the FBI's informant became the sheik's driver.
The FBI began working on the informant while he was in Yemen after 1993.
After the Blind Sheik and nine others were arrested June 24, 1993, the informant still worked for the FBI, sources told NBC News. The mole's success captured the interest of the CIA, which convinced the mole to work for the CIA in 1994.
In 1994 or 1995, the CIA sent the informant to Bosnia, where jihadists were helping Bosnia's Muslim majority in a war against Serbian forces. The FBI did not know its informant had begun working for the CIA, or why he had disappeared, NBC News said.
His former FBI handler, Bassem Youssef, began asking his sources what had become of the driver and was told he had gone to Bosnia.
The former handler also was told al-Qaida operatives in Bosnia killed the man because they believed that he was a CIA mole, NBC News said. Later, the former handler confirmed the al-Qaida operatives' suspicions that the driver had been working for the CIA.
The existence of the informant was first revealed during courtroom testimony in 2010, as part of a discrimination suit Youssef filed against the FBI, accusing the bureau of passing him over for promotion despite his skills, NBC News said.
Testifying on Youssef's behalf, Ed Curran, former assistant special agent in charge of the FBI's Los Angeles office, said Youssef developed the Blind Sheik's driver as an informant.
"It was the only source I know in the bureau where we had a source right in al-Qaida, directly involved," Curran told the court, according to excerpts of testimony published by the Washington Times.