On September 11, 2001, 19 terrorists associated with al Qaeda hijacked four commercial planes, crashing two into the World Trade Center towers in New York City, one into the Pentagon building in Washington D.C., and a fourth into a field in Pennsylvania. Nearly 3,000 people were killed during the 9/11 terrorist attacks, which sparked America's involvement in the War on Terror. The attacks had profound physical and economic impacts, especially in New York City, and resulted in extensive changes to global security procedures.