DURING A PREFLT WX BRIEFING, THE PLT WAS ADVISED OF THUNDERSTORMS & INTENSE RAIN SHOWERS ON THE ROUTE OF FLT. HOWEVER, HE FILED AN IFR FLT PLAN & TOOK OFF. DURING FLT, THE ATC CONTROLLER ADVISED THE PLT OF A LEVEL 6 CELL AHEAD & NOTED THAT HE HAD NO REPORT ON ITS TOP. THE PLT REPORTED THAT HE WAS 'NOT SHOWING ANYTHING' ON HIS RADAR. ATTEMPTS WERE MADE TO DEVIATE. APRX 20 MIN LATER, THE PLT REPORTED THE ACFT WAS ENCOUNTERING ICING CONDITIONS & WAS LOSING ALT. AFTER ANOTHER 1 MIN & 38 SEC, HE REPORTED 'WE'RE GOING IN.' SUBSEQUENTLY, AN IN-FLT BREAKUP OCCURRED. THE WRECKAGE WAS FOUND SCATTERED OVER A 1/2 MI AREA. AN EXAM REVEALED THAT THE OUTBOARD WING PANELS HAD FAILED IN FLT. WITNESSES REPORTED SEEING EITHER SMOKE OR FIRE IN FLT BEFORE THE ACFT CRASHED. EVIDENCE OF AN ELECTRICAL DISCHARGE WAS FOUND ON THE OUTBOARD PORTION OF THE RIGHT WING. HOWEVER, THE PLT HAD NOT MENTIONED ANY ENCOUNTER WITH LIGHTNING ON THIS FLT. NO EVIDENCE OF METAL FATIGUE WAS FOUND.