AT 1450 CDT, WHILE THE PLT WAS CRUISING AT 11,000', ARTCC BROADCAST INFO CONCERNING SIGMET 17C, WARNING PLTS OF THUNDER-STORMS WITH TOPS TO FL 450. AT 1613, RADAR DATA SHOWED THE ACFT AT 11,300'. WHEN ASKED ABOUT HIS ALT, THE PLT REPLIED HE WAS IN A CELL & WOULD BE RETURNING TO HIS ALT OF 11,000'. HE DID NOT REQUEST ASSISTANCE. SHORTLY AFTER THAT, RADAR DATA SHOWED THE ACFT HAD LOST 1000', THEN THE DISPLAY WENT INTO A COAST TRACK. LATER, THE WRECKAGE WAS FND SCATTERED OVER AN AREA APRX 1 MI LONG & 1/4 MI WIDE. THE WINGS & STABILATORS HAD SEPARATED DURING AN IN-FLT BREAKUP. THERE WAS EVIDENCE THE WINGS HAD FAILED FROM POSITIVE OVERLOADS. NO PRE-ACDNT PART FAILURE OR MALFUNCTION WAS FND. THE PLT'S BODY WAS FND AT 0737 CDT ON 10/24/85. A TOXICOLOGY CHECK SHOWED AN ALCOHOL LEVEL OF 0.031%, BUT THE AMOUNT OF ALCOHOL DUE TO PUTREFACTION WAS NOT VERIFIED. A WX SUMMARY SHOWED THE ACFT WAS IN THE VICINITY OF A VERY STRONG WX ECHO WITH MODERATE TO SEVERE TURBULENCE & UP/DOWNDRAFTS IN THE THUNDERSTORM BELOW 20,000'.