This two-volume encyclopedia on the September 11th attacks of 2001 provides in-depth coverage of the participants in the September 11th plot, the sequence of events on the day itself, the reasons American intelligence and law enforcement failed to prevent the attacks, and the various conspiracy theories that have emerged in subsequent years.

For librarian Atkins (Texas A & M U.), an extremist movement, group, or organization deviates from normal economic, political, religious, or social standards; and an extremist individual pursues polices or practices that are far outside the societal norm of personal behavior. He excludes criminals who merely seek personal gain. The period he considers ranges from the founding of the Ku Klux Klan in December 1866, to the December 2001 arrest of Clayton...