A witness, who was also a pilot, reported that he observed the accident airplane on short final approach. The airplane was in a crab attitude due to a gusting crosswind and the witness heard the engine power increase several times. As the airplane neared the runway threshold, it drifted right of runway centerline and slowed, followed by an aerodynamic stall about 100 ft above ground level and descent nose-down into a grass area about 100 ft prior to the runway threshold. The witness went to the wreckage to assist the pilot and the pilot stated that he lost all control of the airplane. The airport manager subsequently confirmed flight control continuity from the cockpit to all control surfaces. After several requests, the accident pilot failed to complete and return a National Transportation Safety Board Pilot/Operator Aircraft Accident/Incident Report form.