According to an April 15, 2002, article in The Washington Post, a tape of bin Laden was discovered in Afghanistan in November 2001. On the tape, “He accurately listed the nationalities and sometimes the names of the nineteen-man suicide squad, all but four his Saudi compatriots. Osama confirmed the existence of four hijacking teams, rejoiced in the tradecraft that deliberately kept each in the dark about the others’ existence. He explained their division between the four witting pilots and the Saudi ‘muscle’ who, he chuckled, learned the exact nature of their suicide mission only ‘just before boarding’ the four airliners to cow their passengers into submission with box cutters once the plane was in the air. Osama said he knew five days in advance that the operations would take place on September 11 and had a radio tuned in ready to hear the first plane hit the Trade Center’s north tower. …He recounted that in the planning stage his engineering training had helped him calculate the number of likely deaths from the explosive impact of a nearly fully fueled airliner on the twin towers’ metal structures. He acknowledged his surprise that they collapsed completely. ‘All that we had hoped for,’ he allowed, was the destruction of ‘three or four floors’ where the aircraft hit and those above the impact.” [The 15th of the month used for date sorting purposes only.]
– Jonathan Randal, Osama, Pages 9-10