THE AEROMEDICAL EVACUATION (EMS-MEDEVAC) FLT WAS MADE AT NGT OVR MTN TRRN. THE PLT INTERRUPTED HIS NORMAL PSN RPRT & BROADCAST 'MAYDAY' 3 TIMES, GIVING HIS PSN TO A COMPANY DISPATCHER. HE DID NOT STATE THE NATURE OF THE EMERGENCY. NO FURTHER COMM WAS RECORDED. WITNESSES IN A SOUTHBOUND AUTO SAW THE HELICOPTER FLYING TOWARD THEM FROM THE SOUTH. THEY STATED THE HELICOPTER'S FUSELAGE STARTED ROTATING COUNTER-CLOCKWISE. IT THEN VEERED TO THE WEST, WENT OUT OF SIGHT, AND CRASHED. ALL MAJ COMPONENTS, EXCEPT ABOUT 33 INCHES OF THE TAIL ROTOR DRIVE SHAFT & ITS #5 BRG, WERE LOCATED. THE TAIL BOOM WAS FOUND IN 3 SECTIONS. ROTATIONAL & LONGITUDINAL SCORING DAMAGE WAS NOTED ON THE TAIL BOOM IN THE VCNTY OF THE #3 & #5 BRG SUPPORTS. A LAB RPRT STATED THAT THE ROTATIONAL DMG OCCURRED BEFORE THE GROSS DEFORMATION OF THE TAIL BOOM & SUBSEQUENT IN-FLT BREAKUP OF THE HELICOPTER.