This was set on fire.

Yep, I agree. I was immediately suspicious when before the first investigator went into the building, the govt. was already declaring "Not arson!"

People are giving all sorts of explanations, from "a worker's blowtorch" to "oily rags spontaneously combusting." I call BS. Blowtorch, really? What were they doing? Soldering a drain pipe? The fire started in the ceiling timbers. Also, the rags...yes, rags damp with flammable liquids, such as varnish or solvents can combust, but I reckon the very best craftsmen were employed on the job and they would know about that hazard and would have taken anything like that with them when they left for the day.

I just posted a link to a Twitter acct. with some suspicious video, but I cannot find any other verification as to its validity or when it was taken.