At 1518 Pacific time, the aircraft broke up in flight and impacted terrain in a nosedown attitude. The airplane was destroyed and the Private pilot/owner and CFI aboard were fatally injured. Visual conditions prevailed. Witnesses, who saw them boarding at Modesto, indicated that the CFI was seated in the left seat, and the pilot/owner was seated in the right seat. Subsequently, the airplane entered the traffic pattern for landing. Witnesses disagree on details, but all agree one wing bent up perpendicular to the fuselage and folded back. The airplane started to spin, pitched down between 60-70 degrees and impacted the ground.