Considering the weight of the turbines and the speed at which they rotate, when they break apart and throw their blades, if the engine housing does not contain them, all types of damage can be done to the rest of the plane, including cutting open fuel tanks and starting fires.
I agree with all those who say that in these times we have to look at terrorism of some type as the cause. But, everything so far physically described could be a result of a turbine failure.
The black boxes will probably answer the questions. Since the remainder of the plane went straight in, the black box in the nose may not have survived. But the one in the tail probably did. That one won't have cockpit conversations, but it will have flying and mechanical parameters. That may be enough to answer the questions.
Do not be surprised if there is a claim by al Qaeda that "we did this," whether or not they did. We can't do any more to them than we are already doing, and a false claim by them that this was their operation would spread the results of terror just as much as if the claim were true.
So I think we need to rely on two things: 1) that they find and can use the black box from the tail, and, 2) that we now have a President who will tell us the truth, when he know it.
And as a former physicist, I agree with those who noted the lag of the speed of sound against the speed of light. The fact that witnesses "saw" parts come off the plane before they "heard" an explosion doesn't answer the question of which happened first. Commercial airliners fly past the mountain I live on all the time. The sound of their engines trails about 25 degrees behind the point where I see the planes.
Conressman Billybob