On the morning of September 11, 2001, four commercial airliners were hijacked by 19 men linked to al-Qaeda in a coordinated attack against the United States. Two planes, American Airlines Flight 11 and United Airlines Flight 175, were flown into the North and South Towers of the World Trade Center in New York City, causing both skyscrapers to collapse within hours. A third plane, American Airlines Flight 77, struck the Pentagon in Arlington, Virginia, while the fourth, United Airlines Flight 93, crashed into a field near Shanksville, Pennsylvania, after passengers attempted to retake control. Nearly 3,000 people were killed, marking the deadliest terrorist attack in modern history and reshaping global politics, security, and the course of the 21st century.