Or a careless contractor.
I used to be an arson investigator, and it's pretty much a given that anything involving ANY kind of repair or renovation work is probably negligence on the part of a contractor.
Most common cause of a fire during a renovation is a careless plumber using a torch on copper pipe without taking proper precautions. A lot of the wood in this structure is old, probably heart pine. If a plumber was sweating a pipe with a torch without an asbestos mat behind his work, he ignited a section of wood. It'll smolder for hours and then burst into flame.
It's doubtful a contractor was there working on the building at 2AM, when the fire broke out. Especially on a Saturday night slash early Sunday morning. Strange.
On a Saturday night/Sunday morning?