There are a limited number of logically possible explanations of some fact or event (as only a limited configuration of other facts, objects or events can produce the fact or event in question). IFF you could change the foundation of observation or reasoning (such as positing solipsism or immaterialism or phyrronism), you could generate a theoretically infinite list of possible alternate explanations, but you've already indicated that you believe (I agree with you) there is apodictic proof for some type of ontological realism / epistemic rationalism. Thus there are a limited number of logically possible explanations of 9/11.

One logically possible explanation is that the events of 9/11 were orchestrated and-or perpetrated by members of the US government. To legally prove guilt in this country you need to establish 4 facts:
corpus delicti - the proof that a crime occurred
means - the ability of the accused to commit the crime
mens rae - motive to commit the crime
opportunity - circumstances allowing the accused to commit the crime

The corpus delicti is generally undisputed (though I wouldn't be too surprised if some anti-America propaganda somewhere denied that 9/11 even happened).

There is certainly the possibility of a mens rae (anything from gaining a strategic military position in the middle east to gaining oil rights).

The means of the US government to commit 9/11 is generally undisputed (government employees who appear on TV generally say things like "the government isn't competent enough to do something like this"--right, we can send R/C buggies to mars, but not secretly demolish some buildings, heh).

The opportunity of the US government to commit 9/11 is generally undisputed.

Thus is is perfectly logical to believe that the US government may have orchestrated and-or perpetrated the events of 9/11. It may not be correct or the most reasonable belief, given all of the evidence, but it is certainly not crazy.

NB. Crazy people may also believe it for crazy reasons, like thinking that giant reptilian aliens from Zeta Reticuli run the US government, but that just makes them and their reasoning crazy, not necessarily what they believe. Just like a person can believe that GWB are the president's initials, and be right, but also believe the president is a big rabbit named "Giant Whiskered Bunny," and be crazy.