On September 11, 2001, terrorists attacked the Unites States. They hijacked four airplanes in mid-flight. The terrorists flew two of the planes into two skyscrapers at the World Trade Center in New York City. The impact caused the buildings to catch fire and collapse. Another plane destroyed part of the Pentagon (the U.S. military headquarters) in Arlington, Virginia. The fourth plane crashed in Shanksville, Pennsylvania. Officials believe that the terrorists on that plane intended to destroy either the White House or the U.S. Capitol. Passengers on the plane fought the terrorists and prevented them from reaching their goal. In all, nearly 3,000 people were killed in the 9/11 attacks.