The airplane was substantially damaged when its left main landing gear retracted on landing. The Airline Transport pilot and the two medical crewmembers aboard the aeromedical flight were uninjured. The pilot subsequently reported an unsafe gear indication after the last takeoff; he recycled the landing gear two or three times until the gear appeared to be secured. He then noticed that the airspeed was too low for the airplanes indicated flight configuration. The pilot suspected a landing gear malfunction and returned to his departure point. He then made a low approach to the runway and was told by his maintenance personnel that the left gear was still extended. The pilot cycled the gear down and observed that the landing gear lights were illuminated, indicating that the gear was down and locked. The pilot made a soft field landing onto Runway 32. The pilot let the airplane roll to the end of the runway, and started to turn to the left, when the left main gear retracted. The airplanes left wing came to rest on the runway.