WHEN THE ACFT DID NOT ARRIVE AT ITS DESTN, A SEARCH WAS INITIATED. THE WRECKAGE WAS LOCATED 4 DAYS LATER WHERE THE ACFT CRASHED IN MOUNTAINOUS TERRAIN. AN INVESTIGATION REVEALED THERE WAS SEVERE THUNDERSTORM ACTIVITY IN THE AREA AT THE TIME OF THE ACCIDENT. NO RECORD WAS FOUND OF THE PLT RECEIVING A PREFLT WX BRIEFING. RADAR DATA SHOWED THAT AN ACFT (PRESUMABLY N3328F) HAD TRACKED NORTHBOUND TOWARD AN AREA OF RISING TERRAIN WITH THE TRANSPONDER SQUAWKING 1200 (VFR CODE), BUT WITH NO MODE 'C' ALT REPORTING. THE ACFT THEN TURNED EASTBOUND FOR 5 TO 7 MI, THEN BEGAN CIRCLING FOR ABOUT 1 MIN UNTIL RADAR CONTACT WAS LOST. SUBSEQUENTLY, THE WRECKAGE WAS FOUND IN THE SAME VICINITY. AN EXAM REVEALED THE ACFT HAD CRASHED IN A STEEP NOSE DOWN, SLIGHTLY INVERTED ATTITUDE, AT HI SPEED. NO PREIMPACT MECHANICAL FAILURE OR MALFUNCTION WAS FOUND.