Correspondent Hedrick Smith outlines U.S. intelligence and law enforcement's many "missed opportunities" to uncover the Sept. 11 plot. Does a fatal "failure of imagination" explain why no alarm bells went off?

Blitzer was the FBI's chief of domestic terrorism from 1996 to 1998. He reflects on the many factors that enabled the Sept. 11 terrorists to outsmart U.S. intelligence, and questions whether there were warning signs missed by the intelligence and law-enforcement communities.

Chairman of the Senate Select Committee on Intelligence, Graham talks about why the failure to uncover the terror plot was due to a lack of human intelligence resources, as well as poor coordination among intelligence and law-enforcement agencies.

An assistant director with the FBI from 1998 to 2000, Schiliro headed its New York bureau where he supervised counterterrorism investigations, including those of the 1993 World Trade Center bombing and the 1998 U.S. embassy bombings in Kenya and Tanzania. Here, he analyzes Al Qaeda's network and strategy in planning the Sept. 11 attacks and why the U.S. wasn't able to thwart the plot.

This section of FRONTLINE's report "Looking For Answers" (Oct. 11, 2001), offers U.S. intelligence authorities' assessments of the specific reasons for the Sept. 11 intelligence catastrophe, as well as larger flaws in the system -- from bureaucratic obstacles and regulatory constraints to agencies' rivalries, lack of resources, and poor coordination in sharing information. It also includes links to significant readings on U.S. intelligence and counterterrorism efforts.