I was a fire investigator for 6 years. We go about finding what started a fire by A) looking at burn patterns and B) interviewing the people that were there if possible.
If the whole house has not burned down, I look for the that has the most burn damage.
In a urban area where the fire department is pretty responsive, you may only get smoke damage in most of the house.
This rules out the rest of the house from starting the fire. Let’s say the most burnt area is near the kitchen. You start asking yourself what can start a fire in that area.
Usually this would be your stove, electrical systems, candles etc.
The next thing you would do is rule out each possible source of fire by looking at witness marks.
These are distinct marks where the flames appeared to originate.
Eventually you will get to a point where you can conclusively rule out what definitely did not cause the fire and hopefully left with one source of the fire.
It is not uncommon to sift through an entire room of debris to find common sources of ignition.
When sifting, we have found cigarette butts, arced wires, matches, and one time an 85 year old woman’s stash of condoms.
If we believe arson was a possibility, we will take samples of the area for chemical analysis.
Arson is pretty obvious as it leaves what is called a ghosting pattern on the floor.
We can pull samples from the edges of the burn and test for gasoline etc.
When it comes down to it, all fires need fuel, oxygen and a source of ignition.
This is called the fire triangle. Our job is to look for the source of ignition.