On September 11, 2001, nineteen terrorists who were members of al-Qaeda, an Islamist extremist network, hijacked four California-bound commercial airplanes shortly after their departures from airports in Boston, Massachusetts; Newark, New Jersey; and Washington, D.C. In a coordinated attack that transformed the planes into weapons, the hijackers intentionally flew two of the planes into the Twin Towers of the World Trade Center, and another into the Pentagon in Arlington, Virginia. Learning about the other hijackings through telephone calls, passengers and crew members on the fourth plane launched a counterattack. When the passengers attempted to breach the cockpit, the hijacker pilot, who had changed course toward the nation's capital, crashed the plane into a field in Somerset County, Pennsylvania, near the town of Shanksville.

Nearly 3,000 people were killed on that day, the single largest loss of life resulting from a foreign attack on American soil, and the greatest single loss of rescue personnel in American history. Approximately two billion people, almost one third of the world's population, are estimated to have witnessed these horrific events directly or via television, radio, and Internet broadcasts that day.