@quentinc:
I don't see how you arrived at that surmise ... are you supposing that because damage is extensive, the origin of the fire is not small or simple?
It's in the nature of fire that small and mundane origins (e.g., a cigarette) can lead to vast destruction.
One of the important questions for safety: had the same situation occurred in flight, in what time frame would it be likely for the fire to be detected and (if feasible) suppressed. This won't be answerable, until the origin has been identified.
If the fire likely would have been stopped early in its evolution, then its safety implications would be less worrisome.