Cornell study finds healthy adults who were close to the world trade center during the terrorist attacks on sept 11 2001 have less gray matter in key emotion centers of their brains may 27 2008. September 11 attacks series of airline hijackings and suicide attacks committed in 2001 by 19 militants associated with islamic extremist group al qaeda against targets in the united states the deadliest terrorist attacks on us soil in us history over 3000 people died in the attacks and rescue efforts. Read cnns september 11 fast facts and learn more about the terrorist attacks on september 11 2001. On 11 september 2001 known as 9 11 in america islamist extremists hijacked four planes that were flying above the us two of them were flown into the twin towers of the world trade center in