During arrival on a dark night, the aircraft collided with level ground approximately 1 mile northeast of the airport. It touched down on a west northwesterly heading in approximately a 45 degree, left bank, slightly nose down, attitude. An exam of the wreckage was made, but no evidence of a preimpact failure or malfunction of the aircraft was found. According to local authorities, the pilot was holding an oxygen mask in his left hand when the wreckage was located. The oxygen system was found in the 'on' position. The oxygen plug in cover for the pilot was off and broken. According to persons who flew with the pilot on a regular basis, the oxygen masks were normally carried in a cardboard box in the baggage area. They reported that even though the oxygen system was serviced, the pilot had never used it and never flew at altitudes where oxygen was required. The pathologist, who performed the autopsy, believed the pilot suffered a coronary, crippling condition. However, he reported the cause of death was blunt force trauma which was suffered in the fatal crash.