Didn’t turn the torch off??
Dumbass is right.
There are a lot of them out there. In the Eighties, a modified bitumen roof was being installed on an old cold-sotrage building converted into loft art galleries. It was a torch-down system in which the edge of successive sheets are “glued” together by heating the edges with a torch and pressing down. One roofer left his torch on as he was getting another roll of roofing; the blaze burned down an entire block. The following day the skeleton of heavy timber bow trusses and columns were still giving off smoke despite being sprayed by fire hoses.