Fallacy of the Single Cause
A well known, and often committed, logical fallacy is the false dilemma. When you consider two variables (US Government, Al Qaeda), you have the following, logically possible options:
|US Government||Al Qaeda|
|Involvement in 9/11|
That's four possibilities, not "either the US did it, or Al Qaeda did it". As you'll note, "neither" and "both" are also options.
What happens when we add Israel, Saudi Arabia and Pakistan?