you are right. Now that I recall, that is how they did it at Waco. Put in the gas and started the fire and then accused Koresh of burning the place down. and Immediately cleaned the scene.
They were in a big hurry to bulldoze the Federal Building crime scene in OK City, too. For a site where there allegedly had been unexploded ordnance in the building (Explosive devices were used as the reason to pull back rescuers on at least one occasion, and, iirc, two or three times), it just seemed a mite hasty and something which would destroy evidence—not the actions of someone who is trying to get to the bottom of a crime.