AFTER TAKEOFF ACFT HEADED TO DESTINATION ARPT LOCATED 4 MILES SOUTH. WHEN APRX TWO MILES OUT, THE PLT TURNED BACK TO THE DEPARTURE ARPT. WITNESSES REPORTED THE ACFT WAS IN A SHALLOW DESCENT, LOW AND FAST WHEN IT COLLIDED WITH TREES AND EXPLODED ABOUT 1400 FT SHORT OF THE RWY. THE CABIN DOOR WAS FOUND ABOUT 40 FT IN FRONT OF THE FUSELAGE. NEITHER THE HANDLE NOR THE LOCK PLUNGER WERE IN THE 'LOCKED' POSITION. EXAM OF AIRFRAME, POWERPLANTS, PROPS, FLIGHT CONTROLS AND FUEL SYSTEMS REVEALED NO EVIDENCE OF PRE-IMPACT MALFUNCTION OR FAILURE. A VIAL CONTAINING 'TOLINASE' 250 MG TABLETS WAS FOUUND AT THE ACCIDENT SITE. A PHYSICIAN STATED HE HAD TREATED THE PLT SINCE 1978 FOR VARIOUS ILLNESSES INCLUDING A HISTORY OF GLYCOSURIA AND, AS RECENTLY AS JUNE 1985, HAD PRESCRIBED THREE TABLETS OF 'TOLINASE' A DAY. 14 CFR 61.53 PROHIBITS THE OPERATION OF ACFT AT ANY TIME MEDICATION IS REQUIRED.