The tenant has now been inside the apartment. The nitrous tank that was thought to have vented actually was still full. The local bomb squad insisted on emptying the nitrous tank for no good reason. There was black powder and other metals in the apartment, but they were in the bedroom that didn't burn.
It now appears that the official ruling will be that the "rocket fuel" hada nothing to do with the fire. The BATFE did visit but didn't find anything that concerned them.
Brian Elfert