What difference does it make if we have "proof" that he is responsible for the 9/11 attacks or not? He was on the FBI's Most Wanted list before that. And he admitted to the first WTC bombing. So, regardless if he did this 9/11 thing or not, he needs to get got.
If we can't get him for 9/11, and we can't prove that, we can still get him for the 1st WTC bombing, Yemen, and the Embassy bombing. Can't prove one crime, use the other ones. That's how they got Capone.