ELi5: How do firefighters and investigators find out what caused a house fire after its already burned to ashes?
In: Other
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.
To start with – visual inspection, deductive reasoning and hypothesis based on prior knowledge and experience. Investigators would then bring in witness statements, dig out the history of a structure, look at similar cases or circumstance, build mathematical models if needed, bring in other experts – especially those specific to the kind of fire or structure in question. Sift through the debris to find clues, what did and didn’t survive the fire, analysis of samples from the scene to identify propellants/fuel/bodies/etc.
It takes a lot of work and effort, and it does rely on experience as much as skill. So to give an idea of how this works [I’m going to use this as an example.](https://ibb.co/4KNkMjz) It’s a picture is of a car outside my place. Trigger for burned car, but no injuries/body parts/blood and gore, and no major injuries or fatalities reported.
I have no witnesses to the fire, only that I know where and when it took place since it was outside my flat. I respond to these incidents to provide healthcare support, not investigate outright, so this is an experiential approach. Got professionally curious so had a look the next day.
*Questions – what can cause the fire that you’ve come across? What can the spread of the fire tell you about the incident?*
First – it’s not completely burned to ash, and even in such large raging fires there will normally be some sort of structure left, or things that were not fully burned away. By seeing how much damage occurred where, especially in relation to what the structure looked like to begin with, you can start to get an idea as to how the fire developed. For this example – there’s clearly the most extensive damage at the engine, with less damage as you go further back – at the rear of the car there is little to no fire damage on the chassis (despite more flammable objects inside the car being burned), so the assumption is that this started as an engine fire.
In this example, what can cause engine fires? High-speed/high-energy impacts – but there isn’t damage to the car’s chassis that would suggest this, as it is mostly intact. Personally even 70 – 90mph collisions I’ve seen/responded to don’t have raging fires like this, even when the battery splits and spills (though when you’re only working with your personal FA kit and mini fire extinguisher you do look very nervously at that battery as you try to evacuate the vehicle safely!). Also this is in a residential area, just after a corner, near a construction site where roads are closed, during morning rush hour where you’re lucky to get 10mph for more than a second. So it is very unlikely to be that. Arson? It is one of the most common reasons to start a fire in the country, however in broad daylight in the middle of rush hour it’s unlikely. Also given the severity of damage to the engine and engine block it doesn’t seem like this would be a fire set by external fuel sources – the volume of fuel needed to have burned outside inwards would have likely spread across the road/pavement/grass an so we’d see more fire damage around the vehicle. This fire looks like it was entirely contained to the vehicle alone. So perhaps a faulty engine? Modern cars are dense with electronics, the fire looks like it burned outwards from the engine itself. I’d need to check the history of this vehicle, and the make and model of the vehicle to see if there are similar reports. I’m ruling out IEDs because there’s no blast damage, there was no bomb disposal team activation, my building wasn’t damaged or evacuated…I’ve got enough lived experiences of IED work to be confident this wasn’t a bomb.
*Question – where are my casualties? (This is pertinent for me as my role in these is to provide healthcare, but we need to be critical of scenes we come across)*
Injuries? Unlikely past superficial. There’s no forced entry/exit from the vehicle as the doors and cabin are intact, the surviving parts of the seatbelt do not look like they were cut/forced/damaged, and I know no-one was admitted as a major burn at the local hospitals due to my work. So even less likely to be a road traffic collision event – in fact I would be confident to rule it out already. Could it be someone getting in, starting the car, it being faulty and sparking a fire? Potentially, plenty of time to calmly leave the car and call 999. But it could still be arson, especially if it was an empty car.
*Question – what do the witnesses say? What did the responders find on initial approach? What was needed to tackle the fire and get it under control? What was found on scene immediately before/during/after the operations?*
From here on in, I don’t know. Wasn’t my job, wasn’t me who responded. For all intents and purposes I am nothing more than a nosy neighbour and since I didn’t even directly witness the event, I’m not even a bystander with information! So those more indepth questions will now be left to those who are working the case.
It’s more a walked initial example than a full explanation, but I hope that starts to help you understand how investigations start?
So, not everything is damaged, and there are clues in the patterns and methods of spreading. Hot gases rise, so ceilings can be very burnt but yhe floor only partially. Generally, fire spreads in predictable ways so you can back-track that to work out the likely “seat” of the fire, which also tends to be where the signs of damage and heat are the strongest.
For example, you find a spot of concrete that shows cracking and looks like it melted, and ceilings above it is extra black and charred. That shows that something was incredibly hot and touching the floor, possibly a liquid fuel like petrol or industrial achohol, or a large pile of cardboard or other flammable material, and thst caused the fire to spread to the celing and the throughout the building.
Unless there was an obvious reason for something like that to be located there, that becomes *potential* evidence of arson. The spread can also point to other things, like a fire door that should have closed being left open, or residue on burnt-out material indicating the presence of materials that shouldn’t be there (improperly stored chemicals, etc).
They don’t.
They make educated guesses based on fires with known ignition sources and from fires done in controlled conditions.
So basically they look for clues and compare those clues to previous situations that match. This isn’t definitive and their reports and conclusions should be taken with a healthy dose of salt, as with all crime scene investigation techniques.
It turns out that different things burn in different ways and leave different damage behind.
So you study that for a long time, then examine a scene, then make an educated deduction.
Latest Answers