New York's Twin Towers were destroyed by two passenger aircraft in the co-ordinated attacks. A third flew into the Pentagon in Washington DC and a fourth crashed in a field in Pennsylvania. Al-Qaeda officially claimed responsibility three years later.
Asked if he was a 'renegade revolutionary' in his post-X Factor career, Cardle told The Big Issue: "Well, there's a lot of conspiracy... I've gotta be careful what I say.
"The whole 9/11 thing... something's not quite right there. I think the way it was portrayed on TV and who was responsible and the way it happened was not the way it happened."
Quizzed on whether the US government led by then-president George W Bush was behind the attacks, Cardle reportedly responded with 'vigorous nodding' and added: "I don't wanna comment!
"But yeah, it's just not what they say it is. Other conspiracies might be true and the one I believe might also be bulls**t.
"But all I do know is what they're saying is bulls**t. From the things I've seen... I don't care what anyone says. To not question it is silly, I think."
Last month, Liverpool Football Club investigated messages on Twitter from striker Nathan Ecclestone claiming that terrorists were not responsible for the September 11 attacks.
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Watch Matt Cardle perform 'When We Collide' live on the X Factor final below