The pilot was cleared for an approach to the airport. He received airport advisories from the airport UNICOM operator and asked that the runway lights be turned up. Soon afterwards, the airplane was seen approaching runway 23, which was served by a localizer approach. A witness at the airport said he saw the plane flying in and out of low hanging clouds. He said the airplane appeared to be at level flight, about 400 to 500 feet AGL, then it passed over the airport and went in a steep left turn. After about 90 degrees of turn, the airplane began a shallow descent. It rolled out of the turn before it descended into the ground about 1/4 mile from the approach end of runway 28. The witness said he heard engine sounds throughout the descent. An examination of the airplane did not disclose evidence of mechanical malfunction. A post-mortem examination did not reveal evidence of impairment or incapacitation of the pilot.