The aircraft crashed into a mountain in a controlled crash situation with full flaps and full power at the 11,312 ft level. The density altitude was computed to be 12,370 ft MSL. The pilot was aware that the weather was questionable in some areas of his route and the mountain tops were obscured. The pilot told the FSS at Grand Junction after a weather briefing that he would go and flight check the weather in the area and call FSS back. After takeoff, there were no further communications with the flight. The pilot gave several accounts of his difficulties to different people, none of which could be verified. The pilot's account of a lightning strike and airframe failure were not evidenced by wreckage examination or by witnesses who saw the aircraft circling the valley west of the trail they were horseback riding on. The pilot said he could not maintain altitude. He turned into the hillside, lowered flaps, applied full power, and settled into the trees. All 4 occupants got out with minor or no injuries.