ELi5: How do firefighters and investigators find out what caused a house fire after its already burned to ashes?
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Most houses do not burn all the way to ashes. There is usually some things remaining. And you might be able to tell from the remains where the fire started. For example looking at a beam you can tell which side have burned longer then the other by how much remaining material there is so you can tell which side of the wall burned the most. And different things burn or melts at different temperatures so you can map out how hot it was in various parts of the room. You may even look at an electrical cover to see if it melted from the inside or outside and you may look at electrical contacts to see signs of wielding from where it short circuited. If accelerants were used these may also leave chemical traces you can analyse for.
Of course none of these are conclusive evidence. A short circuit could happen due to the heat of fire and it is not uncommon to store accelerants in the garage or even in the cleaning closest. So you need to combine these evidence and also take eye witness reports both from neighbours and passers by as well as the first responders. Even then the fire investigation are usually not able to come up with any conclusion. Their conclusion might be good enough to be used in statistics and in some cases even in a lawsuit. However you almost never see fire investigation reports as key evidence in a criminal trial.
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