I agree with Oystein's account mostly, but I'd like to point out some points of disagreement.
I am certain, but allow residual doubt, that flight 93 crashed because its passengers resisted and violently iterferred, thus making the hijacker-pilot lose control and crash. No shoot-down to be sure.
Sabretooth already refined the part about the pilot losing control, and I agree with him that that was not the case. I'd like to point out that I don't have doubts about the passengers revolt and that that's the why of the pilot's maneuvers.
I also have to add that I am uncertain as for who exactly was the pilot of UA93, as it seems that Saeed al-Ghamdi also had piloting skills, seemed to be in the cabin and his name was mentioned in the CVR transcript. But that's a minor detail.
I am convinced, and have no doubt, that the hijackings caught FAA and the military units that were on alert to defend the airspace by surprise, that they did everything they could to handle the situation, but were unable to do anything about the 4 planes. The main reasons for this inability were: Too little time; no contingency plans for such situations, resources too thinly spread; inadequate sharing of information between civilian and military agencies due to inadequate or incompatible technology.
Rather than inadequate or incompatible technology, I believe that the inadequate sharing of information was due to the lack of contingency plans or exercises for such a situation. I don't think that technology, or lack thereof, played a significant role in information sharing. But then I might lack some information on that respect, so I'm open to refutation.
I do not believe that anyone in these agencies knew of the plan and consciously decided not to interfere. I allow for the remote possibility that there are nefarious elements in the agencies who might be so inclined.
While I share Oystein's belief that that was not the case, I don't think the opposite possibility is that remote. In other words, my belief is feeble in that respect.
I am certain that the hijackers were a group of 19 or 20 Arabs, mostly from Saudi-Arabia, who were recruited by Al Quaeda, had the blessing of OBL. Among their leaders were KSM and Mohammed Atta. I admit that there is a little wiggle room for doubt, as KSM was abducted in secrecy and tortured. I want to take information presented by secret services with a good grain of salt.
I am certain that there were 19 Muslim hijackers in action that day. I am not certain about the Al Qaeda link (mostly because it's a part that doesn't raise my interest enough as to make an assessment on the credibility of the sources saying so), but I generally believe it to be the case. I share Oystein's concerns on the declarations of KSM.
I am satisfied with the forensic investigations. They were humongous tasks, had do dive deep into unknown territories, and by and large the efforts were carried out honestly, competently and with sufficient thoroughness. With hindsight, some minor weaknesses may be lamented, such as the failure to investigate WTC7 with the same diligence as WTC1 and 2 were. It is my understanding that the debris of the twin towers was investigated more thoroughly, which undoubtfully is due to its containing so many human remains. There is no foul play.
I have to add the note that the investigations were not directed to determine guilt, but to take corrective actions where needed, to prevent such a disaster in future, and I'm very satisfied with the results.
I also have some concerns about the haste with the recycling, which prevented the investigation from being more thorough. If anything, the worst suspicion it raises on me is the attempt to cover up the asbestos content in some elements, and I don't believe that suspicion to hold any water anyway.
As for WTC7, I also recognize that the pieces were not marked as were the ones from WTC1 and WTC2, therefore making their association with location and specific collapse features nearly impossible. I leave room to consider Charlie Thornton's statements that the NIST report whitewashed poor design or construction of either the towers or WTC7, but I suspect more on the latter.
I fully agree with everything not explicitly clarified here.
