On September 11, 2001, nineteen terrorists who were members of al-Qaeda, an Islamist extremist network, hijacked four commercial airplanes. In a coordinated attack, the hijackers intentionally flew two of the planes into the Twin Towers of the World Trade Center, and a third into the Pentagon. Learning about the other hijackings, passengers and crew members on the fourth plane launched a counterattack, spurring the hijacker pilot to crash the plane into a field in Pennsylvania. Nearly 3,000 people were killed on that day, the single largest loss of life resulting from a foreign attack on American soil.