I do have some firsthand knowledge of fire investigations (but not this one). Insurance and/or fire officials will generally conduct an investigation to understand the cause of the fire. If an electrical fire, it is often possible to determine through forensic evidence, typically through discovery of melted wire/conductors (a fire doesn't typically burn the conductor, just insulation; a short circuit will melt the conductor).
It's not necessary to be licensed, trained, or even have a bit of common sense to install a high-powered audio system in a vehicle. If someone did so without use of a fused link, or some sort of circuit protection, a fire is not at all an unlikely outcome.
I have no knowledge of this specific even. Just offering some general information.