The solo Private pilot was fatally injured when the airplane impacted terrain at approximately 1745 Mountain time. The airplane was destroyed. Shortly before the accident, a pilot-rated witness saw an aircraft come out of a snow squall and fly past him. The aircraft continued to the north for a short period of time and then reversed course. It eventually flew out of sight; the witness then heard a sudden and significant increase in the engines power. The airplane hit a steep slope in the mountainous terrain. The witness reported it was already dark when he saw the airplane; there were low clouds and snow falling in the area.