If you look at a map, you will see that the debris field started with the crash site at north of Shanksville, moved past Indian Lake, and continued to New Baltimore. If an explosion had caused the plane to break up prior to hitting the ground, there would have been debris found before the crash site. As it is now, the debris is all found in the direction of the wind that day. The local paper interviewed a crash expert (I'll look up his name if you'd like) and he said that debris found that far from a crash site is not unusual.