On September 11, 2001, four passenger planes were hijacked by terrorists. Two of the planes were flown into the twin towers of the World Trade Center (WTC) in New York City, causing huge fires that led to the collapse of the towers less than two hours later. One plane crashed into the Pentagon building in Washington, D.C. The last plane crashed in a field in Pennsylvania; it is believed the hijackers had planned to also crash this plane into a building or landmark, but were foiled by the actions of the plane's passengers. Many of these horrific events, including the second plane's crash into the World Trade Center and the collapse of the towers, were witnessed live by millions of television viewers. It was by far the worst terrorist attack on American soil; conservative columnist George F. Will labeled it "the most lethal terrorism in human experience."

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