If this was a slow developing fire, the pilots would have had time to don oxygen masks and do the check list for fighting the fire. That is the expected scenario in a fire that occurs due to an equipment malfunction. This whole thing, from fire and smoke detection to the airplane veering sharply left and then entering a steep right spiral, happened in two minutes. That leads me to think the malfunction was due to some kind of explosion that affected the control system. The smoke and fire detection sent by ACARS would have been incidental to the primary cause of the failure – an explosion. Bomb in a bag in the forward baggage compartment? Bomb smuggled into the blue room by an aircraft cleaner, passenger, or?

I would not rule out an equipment failure completely, but the info so far points to some sort of explosion that affected the control system and set a fire in the area of the forward blue room and cockpit.

Will some Islamic terror group claim credit? So far none has stepped forward, which is kind of unusual but not unheard of.

Just as ISIS did prior to the Russian aircraft atrocity.