19 terrorists hijacked four planes on September 11, 2001 and crashed three of them into important targets - the Twin Towers of the World Trade Center in New York City and the Pentagon in Washington D.C. The fourth plane crashed in a field in Pennsylvania after passengers fought back against the hijackers. Nearly 3,000 people were killed in the coordinated terrorist attacks carried out by the militant Islamic terrorist group al-Qaeda. The attacks triggered major U.S. initiatives to combat terrorism and defined the presidency of George W. Bush.