I don't know what to believe in the whole 9/11 narrative. I do know this though. People don't want to think about it for the most part. Nobody wants to believe that their government would do something like a 9/11 attack on its own people. The government has, however, done things in the past that are similar.
FDR literally allowed the Japanese to bomb Pearl Harbor. Our government has shot down planes and sunk ships that carried civilians in the past, in my opinion hoping that a war would break out. They've sent guns to Mexico that have been used to kill American citizens and then refused to take any responsibility at all for that happening. They've run drugs into the nation in order to pay for programs that they didn't want listed in a budget.
Personally, I haven't really looked at the possibility because, as a libertarian, I'd prefer to touch people with the positive things that libertarianism can bring. I'd prefer to talk about laws that keep us from being as free as the Constitution guarantees. Liberty and freedom are great ideas, and I would rather try to get my few followers that aren't libertarian to think about freedom and liberty over conspiracy theories that will most likely never be proven or disproven.
Would the US government with or without help from other entities knock down a couple of buildings in NYC and kill thousands of Americans, sure it would. Did it.......I don't know, you decide.