Reprising the scene in the White House on 9/11, Mr. Clarke says Dale Watson, the F.B.I.'s counterterrorism chief, called him. "We got the passenger manifests from the airlines," Mr. Watson said. "We recognize some names, Dick. They're Al Qaeda."
Mr. Clarke recalled: "I was stunned, not that the attack was Al Qaeda but that there were Al Qaeda operatives on board aircraft using names that F.B.I. knew were Al Qaeda." Mr. Watson told Mr. Clarke that "C.I.A. forgot to tell us about them."
The above two paragraphs are taken from the first chapter of the Clarke book, which chapter recounts the events of 9/11/01 as they transpired inside the White House. This particular exchange took place, for those who haven't yet read the book, between 9-10 am.
It seems to me that this is a smoking gun. If Clark and Watson were able to identify these people as Al Qaeda that quickly, then it seems to me that with more vigilance they could have been identified before the planes left the ground.
I've never before questioned whether or not these attacks could have been averted, but now I'm not so sure.