The September 11 attacks (also termed as 9/11) were a series of four coordinated terrorist attacks by the Islamic terrorist group al-Qaeda on the United States on the morning of Tuesday, September 11, 2001. It is a black day in the history of America.
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There are also some conspiracy theories regarding 9/11. Some claim that there was, in fact, no plane but rather a cruise missile which hits the WTC.
A retired army major, Albert N. Stubblebine III, once in an interview said he can conclusively prove that the Pentagon was not hit by a Boeing 757.
Another theory which claims that one of the towers would not have collapsed in the way they did if they were hit by aircraft. Therefore it was a controlled detonation.