The images are unforgettable: On September 11, 2001, two airplanes crashed into the twin towers of the World Trade Center, in New York. Two thousand eight hundred people died in that attack alone, and almost 250 others were killed in the related plane attack on the Pentagon and when a fourth plane crashed in Pennsylvania, unable to reach its intended target, suspected to be somewhere in the U.S. capital. The series of attacks, for which the Islamic terror organization Al Qaeda claimed responsibility, changed the world, raising one question over and over again: Why do people become terrorists?