The justice department's case against accused 9/11 conspirator Zacarias Moussaoui is in trouble. The Feds also have a problem in another case. The credibility of the FBI's star witness against a Pakistani-American accused of Qaeda ties is under fire after he testified he saw Ayman al-Zawahiri at a mosque attended by the de-fendant in Lodi, Calif., in 1999. The testimony by informant Naseem Khan, who has been paid $250,000 by the bureau, startled counterterrorism experts: Zawahiri, Osama bin Laden's chief deputy, was in Afghanistan at the time, under indictment for the bombings of two U.S. embassies in Africa. Jack Cloonan, a former FBI agent who had tracked Zawahiri for years, told news-week the testimony was "bizarre" and "seriously wrong." While conceding "there may have been some puffery" by the informant, a Justice official who asked not to be ID'd because of legal sensitivities said prosecutors are still confident about the case against Hamid Hayat, 23, accused of attending a Qaeda camp in Pakistan, and his father, Umer Hayat, 48, who allegedly lied to the FBI about his son's travels. Defense lawyers say the testimony raises new questions about Khan, a fast-food worker with a burglary conviction who was recruited by the FBI after 9/11 and paid up to $4,500 a month to ID terror suspects. An internal FBI doc obtained by NEWSWEEK shows Khan claimed to have seen Zawahiri--and two other most-wanted terrorists--at the Lodi mosque in late 1999, describing them as Pashto-speaking Pakistanis. (They are Arabs.) The FBI first used Khan to target two radical imams in Lodi. They were deported. Meanwhile, Khan befriended the younger Hayat, a fruit packer, and secretly taped conversations while encouraging Hayat to visit a Pakistani camp in 2003. Hayat later confessed going to the camp; there is no evidence he plotted any terror acts.