A Cessna 172F collided with level terrain after making several low passes. The aircraft appeared to be making closed traffic patterns to a field located adjacent to a residential area. The aircraft was observed by several witnesses flying low over their residences. One homeowner identified the aircraft as his brother's. According to family members, the aircraft was observed making several low passes aligned with an open field. They estimated the altitude to be about 100 ft AGL. The sound of several engine power reductions was noted, followed by engine acceleration. The aircraft was aligned for another approach when the 'engine noise was cut.' The aircraft was then observed pitching downward abruptly. The aircraft struck the ground in a nose down attitude. A fire erupted and the airplane was consumed in flames. Examination of the wreckage did not reveal any evidence of mechanical failure or malfunction. The aircraft wing flaps were determined to be in a 5 degree transitory position at the time of impact.