A man stands near the rubble, asking if anyone needs help, after the collapse of one of the World Trade Center towers on September 11, 2011. In what was the worst terrorist attack in U.S. history, 2,753 people were killed when two hijacked planes were intentionally crashed in the north and south towers of the New York buildings. Two other planes were also hijacked: One crashed into the Pentagon in Washington, and one crashed at a field near Shanksville, Pennsylvania.
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