It wouldn’t necessarily require active cooperation and concealment by all those agencies and people. Assuming that the FBI had primary jurisdiction and control over the investigation, it would only take a word from a senior official to the supervising agent to suppress a particular kind of information (such as interviews confirming an object shooting up towards the plane). The supervising agent could collect all the witness interviews and give instructions that he deems them unreliable, etc. Do you think at that point a couple of field agents who did the interviews are going to risk their careers to speak up publicly?
IOW, it is possible that the whole effort to block the terrorist attack narrative could be narrowed down to a small handful of people, all strongly motivated by career ambition not to break ranks.
Possibly. George Stephanopoulos may have let the cat out of the bag on TV back then when he ‘accidentally’ referred to the aircraft being ‘shot down’ ...................