They knew but did nothing… read this excerpt at The Sydney Morning Herald detailing the failure of the Bush administration to assess the CIA reports of an impending attack.
But what had happened in the White House in the first eight months of George Bush’s presidency had all but guaranteed that 19 young Arab men with little more than pocket knives, a few cans of mace, and a misunderstanding of the tenets of Islam could bring the US to its knees.
More on Shenon’s book here