Noam Chomsky has written a blog post denying that there is evidence of Osama bin Laden's responsibility for 9/11. He writes:
In April 2002, the head of the FBI, Robert Mueller, informed the press that after the most intensive investigation in history, the FBI could say no more than that it “believed” that the plot was hatched in Afghanistan, though implemented in the UAE and Germany. What they only believed in April 2002, they obviously didn’t know 8 months earlier, when Washington dismissed tentative offers by the Taliban (how serious, we do not know, because they were instantly dismissed) to extradite bin Laden if they were presented with evidence—which, as we soon learned, Washington didn’t have. Thus Obama was simply lying when he said, in his White House statement, that “we quickly learned that the 9/11 attacks were carried out by al Qaeda.”
Nothing serious has been provided since. There is much talk of bin Laden’s “confession,” but that is rather like my confession that I won the Boston Marathon. He boasted of what he regarded as a great achievement.
(Source: Guernica / Noam Chomsky: My Reaction to Osama bin Laden’s Death)