Among those were several large aircraft including the 1996 ValuJet crash in the Florida Everglades started by improperly marked and stowed oxygen generators. If you suspect a conflagration has broken out, one of your first duties is to discern its origin. Fires can be started and prolonged by a variety of sources. Fuel and oil fires can erupt from broken hoses or lines, but they still need some kind of ignition source. A bit of flashover from the ignition system or contact with a hot exhaust system often is enough to commence the inferno.
Electrical fires can start ahead of the firewall or in the cabin.
The worst are those that are started and sustained by a circuit that either is not protected by a fuse or circuit breaker or one that cannot be broken from the cockpit. Generally, these amount to high-current circuits, such as the battery main, starter supply line and, in some cases, the alternator output. If the alternator breaker is inside the cabin which it normally is there must be a length of its supply wire that is unprotected. It also seeks to protect current-carrying lines downstream as much as possible. Most of the time, it all works.