It is one of the great unsolved riddles of the plot. Was Zacarias Moussaoui - arrested a month before the 2001 attacks while he was taking flying lessons in Minnesota - the intended "20th hijacker" of 11 September?
The 9/11 Report presents evidence he might have been, and that had news of his arrest reached Osama bin Laden in Afghanistan, the whole operation might have been called off.
In its 567-page report, the commission says Ramzi Binalshibh - a close friend of the hijackers' ringleader Mohammed Atta captured in Pakistan in 2002 - assumed Moussaoui was a designated pilot for the operation. He believed the French-Moroccan student had been selected and assigned by Bin Laden to replace Ziad Jarrah, a designated pilot who had quarrelled with Atta and threatened to withdraw from the operation.
The information comes from a US intelligence interrogation of Binalshibh on 7 March, 2003. Binalshibh said had Bin Laden and Khalid Sheikh Mohammed - the main logistical organiser and financier of the hijackers - found out Moussaoui was now in the hands of the US authorities, they might have cancelled, or at least postponed, the attacks.
But they only learnt about his detention after the attacks. Moussaoui was not needed anyway, as Jarrah patched up his dispute with Atta, and was part of the hijacking group for United Airlines Flight 93.
Moussaoui is the only person in the US formally accused of being part of the 9/11 plot. Prosecutors charged him with conspiracy to commit an act of terrorism, and are seeking the death penalty. Moussaouidenies involvement in the attacks.