What makes it difficult to disregard that entirely is the fact that the U.S. has planned false flag attacks before. There's also the fact of the Bush Administration's open desire to start regime change wars and the adjacent awareness that this would be politically unlikely absent "a new Pearl Harbor" (see "Critics"). This is like knowing someone has at least planned a murder before, then them talking about how convenient it'd be for someone to wind up dead, then a body showing up.

Personally, I think it's plausible that the feds had specific advance warning of the attacks but intentionally slow-rolled the response (I see some parallels with how all these mass shooters were known to the feds beforehand). This would not require a lot of people to know and has excellent plausible deniability. I don't think it's plausible that they set demolition charges, or fired a missile into the Pentagon, or anything like that -- too many people would have had to be involved and this would have left too many smoking guns.

I don't think it's useful as leftists to get too entrenched on any related topic, besides "why would anyone want to do this to the good wholesome U.S. of A.?" or "is the U.S. government capable of killing its own to start a war, or at least looking the other way and letting it happen?"