The aircraft was routinely flown to Pensacola to pick up a Gulf Power executive for a flight to Atlanta. The pax had 4 bags, 2 placed in the aft baggage area by the ground crew, the other 2 carried onboard by the pax. After takeoff the CVR recorded the pax stating that there was a fire. The fire in the aft cabin area was confirmed by the 1st officer. An emerg was declared to the PNS ATCT. Subsequently, the aircraft was seen descending out of the overcast streaming dark smoke behind it. The aircraft impacted an apartment complex and a post crash fire destroyed it. No evidence of a lightning strike, aircraft system malfunction, or incendiary device was found during the investigation. Forensic chemical tests showed traces of hydrochloric and sulphuric acid on pax articles but none on the cabin interior. Metallurgical exam of broken fuel line showed overload failure and post heat distress. The interior of the cockpit windows were severely sooted. The flight crew did not don available oxygen masks.