The acft was on a parachute jumping mission. After climbing to 9000 ft msl, 4 sky divers jumped out & the plt began a descent back to the arpt. The acft was seen level at aprx 500 ft agl & in the aprx position for a downwind leg for a left pattern for runway 36. About 1/2 mi north of the normal turn point for the base leg, the acft began a steep descending left turn. Witnesses described a bank angle of 45 to 75 deg & a nose down attitude of 20 to 30 deg. One of the witnesses reported that the bank and descent angles then began to decrease, but at impact, the nose was still low & the bank angle was about 20 deg. Observers saw no indication of plt control movement before impact. An exam of the wreckage revealed no evidence of an inflt failure/malfunction. The plt’s wife said that onoccasions, he had experienced severe headaches, but never to an alarming point. She reported that he took only asprin for the headaches. Pathological & toxicological exams reflected no evidence of a preimpact incapactiating problem.