On the morning of September 11, 2001, America suffered the worst terrorist attack in its history when al-Qaeda terrorists launched coordinated suicide attacks against a range of targets on U.S. soil. Having smuggled knives and box-cutters through airport security, 19 terrorists quickly commandeered four passenger jets departing Boston, Newark, N.J., and Washington, D.C. One hour after deliberately crashing two planes into both towers of the World Trade Center in Lower Manhattan, which later collapsed, a third was directed into the west side of the Pentagon in Arlington, Virginia, the headquarters of the U.S. Department of Defense. Passengers on board the fourth plane, United Flight 93, fought to subdue the terrorists after learning of the earlier hijackings, resulting in the plane crashing into an empty field near Shanksville, Pennsylvania. All told, some 2,977 people were killed on that day.