Prior to the attacks on the World Trade Center and the Pentagon, all 19 of the terrorists had been living in the U.S. for several months, some for more than a year. They obtained visas, signed apartment leases, shopped, prayed at mosques, rented cars and bought airline tickets.
Could they have done so — especially those who did not speak English — without help from American citizens? It is one of the top unanswered questions lingering about the 9/11 attacks.
"It seems to me that there was some other support mechanism there,” said Roger Cressey, who was director of transnational threats for President Bush's National Security Council. “Now, did that support mechanism know what these individuals were going to do? We don't know. But I think there was something here in the United States they relied upon."
The 9/11 commission found there was "no evidence" the hijackers received help from U.S. citizens who knew about the plot.
"It is still an open question whether the people in the United States who helped the hijackers were witting or unwitting," former 9/11 commission member Jamie Gorelick recently told “Hardball.”
Analysts point out that just because there is "no evidence" doesn't mean it didn't happen.
"You're exactly into the area where you don't know what you don't know,” said Michael Scheuer, a 22-year CIA veteran. “Which is one of the reasons that when the FBI says, we have no evidence of al-Qaida cells in the United States, it sounds reassuring to Americans, but it basically means we haven't found them yet. To assume that they're not here is an extraordinarily dangerous thing to do."
Many questions have focused on two of the 19 hijackers, Nawaw al Hazmi and Khalid al-Mihdhar.
They were among the first four al-Qaida members Osama bin Laden chose for the plot. Hazmi and Mihdhar settled in San Diego in February 2000, about 18 months before 9/11. According to the 9/11 report, Khalid Sheik Mohammed, who helped organize the attack, "instructed Hazmi and Mihdhar to pose as newly arrived Saudi students and seek assistance at local mosques."
Hazmi and Mihdhar made friends in San Diego. One friend investigated by the FBI — Mohdar Abdullah — was among those students who "appear to have held extremist sympathies." The 9/11 report says he helped Hazmi and Mihdhar get driver's licenses and enroll in English classes.
Shortly after 9/11, Abdullah told the FBI he knew nothing about the plot. Later, while being held on immigration charges, Abdullah reportedly bragged to fellow inmates that he had advance knowledge of the attacks. The FBI could not corroborate the story.
Abdullah was not charged, though he was eventually deported to Yemen.
The 9/11 report declared, "Our inability to ascertain the activities of Hazmi and Mihdhar during their first two weeks in the United States may reflect al-Qaida tradecraft designed to protect the identity of anyone who may have assisted them during that period."
In the months that followed, Hazmi and Mihdhar also befriended another man in California, Anwar Aulaqi, a religious leader who soon moved to Virginia. Eventually, Hazmi himself moved to Virginia and showed up at Aulaqi's mosque.
Along the way, did Aulaqi or any other Muslims know what any of the 19 would-be hijackers were planning?
"I suspect they didn't simply because al-Qaida is very, very good at compartmentalizing information,” Scheuer said. “They don't talk a lot about what they are going to do until after the fact when they explain pretty intricately what they, what they did do. But those boys were there for a while."
The terrorists did have their share of setbacks leading up to 9/11. Hazmi and Mihdhar were supposed to improve their English and learn how to fly. They failed.
Still, on 9/11, they boarded American Airlines Flight 77 and helped three other terrorists seize control of the cockpit. Then, the hijackers crashed the aircraft into the Pentagon.
Again, the 9/11 commission says there is "no evidence" the hijackers received help from anybody in the U.S. who knew about the plot. But it's a question that has long bothered investigators.
Investigators are also convinced that there was supposed to be one more hijacker involved in the plot. This is the so-called "20th hijacker." We will examine the questions about who that may be in our third report on Friday.
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