The 9/11 terrorist attacks (AP Photo/David Karp)

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On the morning of 11 September 2001, al Qaeda terrorists hijacked four planes, crashing two into New York's Twin Towers, one into the Pentagon, and one - headed for Washington DC - crash-landed in Pennsylvania. These are the defining images of that tragic day.

At 8.46am, American Airlines Flight 11 crashed into the north face of the North Tower of the World Trade Center in New York City.

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