In one of the most horrifying attacks ever against the United States, terrorists crashed two hijacked airliners into the World Trade Center on Sept. 11, 2001, in a deadly series of blows that brought down the twin 110-story towers. Another hijacked plane slammed into the Pentagon as the government itself came under attack. A fourth plane crashed near Pittsburgh, it's target suspected to be Camp David, the White Hose or another Washington, D.C. location.

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