The building was under renovation. Some worker prolly left a torch on.
My money's with construction worker error. There are SO many things around a construction site that can cause a fire - careless plumber igniting a beam that smolders, space heater left on, unfinished wiring job short circuiting, tossed cigarette on all the McDonald's bags they stuff down the heating ducts . . .
I've personally investigated fires with those causes . . . yes, including the McDonald's bags (if you're smart, when you buy a house you'll take off the duct covers and look in there . . . yecch!)