From the government itself? You can listen to unclassified / declassified 9/11-related communications (all over YouTube) and literally hear the conversations happening. The pilots have done interviews. There’s really no questioning it. It’s part of the US being woefully unprepared for 9/11 because it was so left of field. All evidence points to a lot of brave people doing their absolute best, working within a system that didn’t know how to handle the circumstance. That includes not being prepared to take down a commercial airliner at short notice, because why the hell would anyone ever need to do that!? (no sarcasm intended).
How has this claim been authenticated?