Report of the 9/11 Commission

Some of the vulnerabilities of the plotters become clear in retrospect. Moussaoui aroused suspicion for seeking fast-track training on how to pilot large jet airliners. He was arrested on August 16, 2001, for violations of immigration regulations. In late August, officials in the intelligence community realized that the terrorists spotted in Southeast Asia in January 2000 had arrived in the United States.

These cases did not prompt urgent action. No one working on these late leads in the summer of 2001 connected them to the high level of threat reporting. In the words of one official, no analytic work foresaw the lightning that could connect the thundercloud to the ground.

As final preparations were under way during the summer of 2001, dissent emerged among al Qaeda leaders in Afghanistan over whether to proceed. The Taliban's chief, Mullah Omar, opposed attacking the United States. Although facing opposition from many of his senior lieutenants, Bin Ladin effectively overruled their objections, and the attacks went forward.