there are always the extremists on both sides in the US that have some conspiracy storys. The various ones include
1. the Israelis did it
2. George Bush 1, friendship with saudi’s means Bush set up to get more power.
3. Saudi’s Told Bush, Bush let it happen to get more power
4. CIA did it to get more power for bush.
I think most of the people who live in the US pretty much accept the explanation that there is a group of terrorist led by bin laden who flew planes into the world trade center, pentagon and were likely headed to hit Capitol.
At the time it was easy to pull off because the people on the planes thought it was a hijacking and reacted by assuming it was best to behave, let them land the plane and have the authorities save you. The “Hijacking Model”. As we saw with the one plane where they learned what had happened, they fought back and prevented the mission from succeeding.
People love conspiracy theories, but the one being presented as “what really happened” by the 9-11 commission is what seems most likely. These same people bombed the world trade center once before and car bombed US embassies in Kenya and Tanzania in the 1990s. Who needs a better “Theory” than that? 
Oh, i know a few who wonder if not fully believe that stuff. But most people i know in US and Canada believe the general narrative.