At about 1130 Pacific time, the airplane collided with terrain while maneuvering for landing. Visual conditions prevailed. The aircraft was substantially damaged; the Private pilot and his passenger were fatally injured. Witnesses reported the aircraft was at about 300 feet agl in the pattern for landing and close to the runway during the downwind leg. The airplane then made a steep banking 180-degree left turn to final when it descended left wing low, colliding with trees and the terrain about 200 yards short of the runway. Witnesses reported hearing the engine during the entire maneuver.