Investigation. Your police and/or fire department may provide one, however, it sometimes helps if you hire a private investigator, especially if you disagree with the official report. Does your building have security cameras? If so, contact them at once to request they save the footage! (The police might already have done so but make sure. Video systems often re-record over the tape within 24 hours or so.) I have gone through this with an electrical fire at one of my rental houses (the fire in turn blew out the utility's transformer on a nearby power pole, for which my insurance had to pay).