Two years after al Qaeda terrorists slammed jetliners into the World Trade Center and the Pentagon, FBI and congressional investigators remain deeply divided over whether the 19 hijackers received help from other al Qaeda operatives inside the United States and still are unable to answer some of the central questions in the case.
The uncertainties persist despite the largest FBI investigation in U.S. history -- which has included 180,000 interviews and 7,000 agents -- and raise the possibility that Americans will never know precisely how the conspirators were able to pull off the most devastating terrorist attacks in U.S. history.
"We know quite a bit about the attacks," FBI counterterrorism chief Larry Mefford said last week. "Unfortunately, we don't know everything."
Some of the doubts surround intriguing details: Investigators still have no firm grasp on why the hijacker pilots booked layovers in Las Vegas during apparent practice runs on commercial airliners in 2001. Authorities also have found no definitive explanation for why ringleader Mohamed Atta and another hijacker, Abdulaziz Alomari, began their suicidal journey on Sept. 11, 2001, with a seemingly risky commuter flight from Portland, Maine, to Boston -- coming within minutes of missing their flights out of both cities. And what exactly was discussed at a pivotal meeting in Kuala Lumpur in January 2000, where investigators believe -- but cannot prove -- that the Sept. 11 plot was put in motion?
But perhaps the biggest riddle -- one that has only become murkier in recent months -- centers on the support given to the hijackers while they were laying the groundwork for the attacks, and what that suggests about a pre-existing network of operatives in the United States.
A recent congressional inquiry raises the possibility that al Qaeda supporters were in place in this country to help the hijackers; were aware of at least some aspects of the plot; and may have been supported by elements of another government, Saudi Arabia. If true, that could mean that domestic accomplices to the attacks are still at large.
FBI investigators -- who initially believed that such a support network was likely -- concluded by early 2002 that no evidence could be found of any organized domestic effort to aid the hijackers. Since then, FBI, Justice Department and intelligence officials have portrayed the hijacking teams as disciplined operatives who kept to themselves and did not draw upon existing terrorist cells for help. Investigators believe the hijackers relied on unwitting fellow immigrants in obtaining apartments, identification papers and other assistance after they had entered the United States.
"While here, the hijackers effectively operated without suspicion, triggering nothing that would have alerted law enforcement and doing nothing that exposed them to domestic coverage," FBI Director Robert S. Mueller III said during a joint inquiry of House and Senate intelligence committees in June 2002. "As far as we know, they contacted no known terrorist sympathizers in the United States."
But in a scathing report released this summer, the joint inquiry reached a much different conclusion: that intelligence sources and the FBI's own investigation had revealed contacts between the lead hijackers and at least 14 suspected terrorist associates in San Diego and elsewhere in the United States -- including several whom the FBI was monitoring at the time of the contacts. The congressional inquiry also alleged that two of the associates may have had ties to the Saudi Arabian government, a charge that has strained U.S. relations with Riyadh.
The claims refocused attention on the performance and competence of the FBI, which along with the CIA, came under fierce criticism last year for not acting more aggressively to locate two of the hijackers who were known to have entered the United States in the summer of 2001.
"The fact that so many persons known to the FBI may have been in contact with the hijackers raises questions as to how much the FBI knew about the activities of Islamic extremist groups in the United States before September 11," the congressional report concluded, adding that the extent of any support network "is vitally important in understanding the modus operandi of the hijackers and al Qaeda."
Daniel Benjamin, a former National Security Council official in the Clinton administration who has criticized the FBI's role in combating terrorism, said, "The FBI's line for the longest time after 9/11 was that this was a revolutionary act in terms of the history of terrorism because it was done by terrorists who came into this country and did not plug into the local infrastructure. Now it looks like that is not the case."
But officials at the FBI and elsewhere in the Bush administration strenuously dispute those characterizations, arguing that the congressional inquiry's conclusions rely on outdated or inaccurate evidence and contradict the most recent findings in the case.
Officials said all of the alleged associates referred to in the report have been exhaustively investigated. Although some of the key figures appear to have radical Islamic beliefs or ties, there is no evidence of prior knowledge or involvement in the Sept. 11 plot, investigators said.
"The 14 people that they say are so-called associates to the hijackers have been thoroughly investigated and, in some cases, are two and three times removed from any hijackers," one investigator said. "These were people who had some limited contact with people who later turned out to be hijackers; that does not mean they were in on anything or part of al Qaeda."
Eleanor Hill, staff director for the joint House-Senate inquiry, said she remains concerned that the FBI and other U.S. intelligence agencies are "missing the point" of why contacts between hijackers and suspected terrorist associates are important.
"The question shouldn't be, 'Did these people know about the plot?' " she said. "The question should be, 'Were they placed here by al Qaeda to help al Qaeda operatives, whether or not they knew about the plot, and are they still here?' "
For example, Hill noted, the CIA found that al Qaeda lieutenant Khalid Sheik Mohammed, the mastermind of the Sept. 11 attacks, traveled to the United States as recently as May 2001 and had sent recruits here to establish terrorist networks. CIA Director George J. Tenet also told the inquiry that most of the Sept. 11 hijackers may not have known details of their mission.
"It's very consistent for al Qaeda operatives not to know exactly what is being planned, but that does not mean they aren't here or don't pose a threat," Hill said.
The congressional inquiry released to the public in July provides details on approximately half of the 14 associates alleged to have had contact with the hijackers. They include an unnamed individual who took flight training with hijacker Hani Hanjour in Phoenix, and another unidentified person "on the East Coast" who had ties to one of Atta's former college roommates.
But the most controversial allegations, the ones that receive the most attention in the report, center on a cast of characters in San Diego, where hijackers Khalid Almihdhar and Nawaf Alhazmi settled in early 2000. The differing opinions on the events in San Diego illustrate the depth of disagreement between the congressional inquiry team -- which concluded that the hijackers probably were aided by terrorist associates -- and FBI investigators, who have determined that the contacts were essentially innocent.
The most prominent associate named in the report is Omar Bayoumi, a Saudi national who befriended the hijackers and apparently encouraged them to relocate from Los Angeles, where they had arrived in January 2000, to San Diego.
There is great debate within intelligence and law enforcement circles about Bayoumi and whether he had ties to al Qaeda operatives or was, as one source told the FBI, an agent for the Saudi government. The FBI, which recently completed interviews with Bayoumi in Saudi Arabia in reaction to pressure from Congress, has concluded that those claims are without merit and has largely abandoned further investigation, sources said.
One key component of the conflicting assessments of Bayoumi was his initial meeting with Almihdhar and Alhazmi.
According to the inquiry's report, an unnamed source interviewed by the FBI said he traveled to Los Angeles with Bayoumi on Jan. 15, 2000, to visit the Saudi consulate, details of which the FBI has not been able to determine. Afterward, the report said, Bayoumi and the source went to a restaurant, where they struck up a conversation with Almihdhar and Alhazmi after hearing them speak Arabic. The report notes suspicions by FBI agents that the "meeting at the restaurant may not have been accidental," and an FBI written response to the inquiry refers to the encounter as a "somewhat suspicious meeting with the hijackers."
But FBI investigators said that subsequent investigations have erased many of their suspicions. Investigators have determined through interviews that Bayoumi and his companion initially sought out a different Arabic restaurant that had closed and been turned into a butcher shop. The butcher has told the FBI of encountering the pair, and of directing them to the other restaurant, where they met the hijackers.
FBI officials said they also have discounted other suspicious information about Bayoumi, including a claim by one source that Bayoumi delivered $400,000 from Saudi Arabia to a Kurdish mosque in San Diego. While Bayoumi did provide a cashier's check for the hijackers' initial rent payment and security deposit in San Diego, it amounted to nothing more than a "seven-minute loan" that was repaid with a cash deposit into Bayoumi's bank account, one investigator said.
The bureau's Sept. 11 investigative team, which is still tracking down details of the plot, has reached similar conclusions about other associates named or referred to in the congressional inquiry report.
"There is no indication that these people who provided assistance knew what they were up to," said Mefford, the head of counterterrorism and counterintelligence at the FBI. "Most of this assistance is very benign cooperation. . . . Did anyone in the United States know what they were up to? At this point, there is no evidence of that."
Research editor Margot Williams contributed to this report.