The left main landing gear collapsed on landing rollout. Examination of the landing gear by the FAA and company maintenance personnel revealed a failure of left landing gear upper barrel and trunion assembly. Further examination of the components by the NTSB laboratory revealed fractures representative of overstress separations. There was no evidence of fatigue or other type of preexisting cracking. Review of aircraft logbooks revealed no recorded history of hard landings, or previous landing gear damage.