looking at surfaces that were interior to the home and comparing them to surfaces that were exterior but essentially in the same location. If there is a distinct difference between the residue on these surfaces, it would suggest a gas explosion. If on the other hand the surfaces display a similar amount, it would suggest the residue was caused by fire that persisted after the explosion.
For instance, since pieces of sheet rock can be assumed to have fallen in random orientations after the explosion, if it is found that this debris only has residue on the interior surfaces - independent of how they came to rest - the residue was likely caused by the explosion.
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