The fire investigators are looking seriously at arson for this case. No proof yet, but it does not look like an accident to them.
There is no question that there can be no more fire-vulnerable moment in a construction project of this sort than you have bare wood framing, no sheetrock, no alarms. A full block of kindling, dry wood.
But the fire dep’t is across the freaking street! For this thing to completely envelop in fire, IMO it had to be accel’ed w/flam liquid at several locations. This wasn’t a homeless guy cooking some sausages or stripping off some copper wire.