On May 11, 2000, at about 1625 central daylight time, a Cessna 172H, N5107G, registered to a private owner, operating as a 14 CFR Part 91 personal flight, experienced an in-flight fire in the vicinity of Picayune, Mississippi. Visual meteorological conditions prevailed and no flight plan was filed. The airplane was destroyed by fire. The private pilot and one passenger reported no injuries. The flight originated from New Orleans, Louisiana, (NEW) about 25 minutes before the accident. The private pilot stated that he departed NEW at about 1600, for the VFR local flight. He was over flying the Picayune Airport at 2,500 feet when he observed smoke coming up from behind the firewall followed by flames. He immediately turned the fuel selector valve off and made a forced landing to the runway. He and the passenger exited the airplane uninjured. The airplane was consumed by fire. The pilot stated in the NTSB Pilot/Operator Aircraft Accident Report that he dropped off his friend at the Picayune Airport, took off, and experienced the smoke in the cockpit at about 500 feet. After making a safe landing at the airport, he got out of the airplane and ran a safe distance away from the plane. Statements provided by the pilot or his passenger to the Pearl County Sheriff Department does not substantiate the pilot's statement. (For additional information see Pearl County Sheriff Department Report, an attachment to this report.) Examination of the airframe, flight controls, engine assembly and accessories by the FAA and parties to the NTSB investigation could not determine the source of the reported in-flight fire. (For additional information see Aircraft Mishap Report Cessna Aircraft Company, Teledyne Continental Power plant Examination, and FAA Inspector Statement ttachmentS to this report.)