The 9/11 attacks refer to a series of coordinated terrorist attacks that occurred on September 11, 2001, in the United States. Four commercial airplanes were hijacked by members of the extremist group al-Qaeda. Two of these planes were flown into the World Trade Center towers in New York City, causing both buildings to collapse. The third plane struck the Pentagon in Arlington, Virginia, while the fourth plane, United Airlines Flight 93, crashed into a field in Pennsylvania after passengers attempted to regain control from the hijackers. Nearly 3,000 people lost their lives in these attacks, which had a profound impact on U.S. and global security policies.