By an odd coincidence I recently read a book called
Air Accident Investigation by David Owen. This is not a text or technical book but a general-audience one and it consists largely of histories of airplane crashes and how investigators worked out the causes of each. In the first chapter Owen gives some examples of how investigators draw inferences from wreckage, damage to objects on the ground and so on- tidbits like how an accident investigator can determine if an indicator or warning light in the cockpit was on or off when the plane impacted. One passage that seemed relevant to the crash of flight 93:
The bolding is mine, to pick out items relevant to the Shanksville crash site. Please to note that Owen speaks of a "smoking hole impact" in a way that indicates that this is a phenomenon familiar to accident investigators, as are the clues provided by the smoking hole itself.