Wasn't the Courthouse re-built from another fire? I don't think it was original from the BTTF shoots.
They keep saying it was the video vault, yet they showed film cans on the news just now being removed.
I bet they the film masters are in a vault in the desert for safe-keeping anyway. Still, losing a video vault can cause a lot of money and time wasted. This is where you would keep a lot of TV back-ups and foreign mixes on D5, etc. Not the end of the world because it can all be re-done.
They were saying here that it could be the result of a commercial shoot explosion, but I can't find any back-up on that yet to post.
The small mill is nearly lost, too.