THE ACFT ENTERED AN EMBEDDED THUNDERSTORM AFTER BEING ADVISED BY ARTC OF SEVERAL LARGE AREAS OF HEAVY PRECIP IN HIS AREA& ALONG THE VICINITY OF HIS FLT PATH. THE PLTS RESPONSE WAS THAT HIS ACFT RADAR SHOWED NO BUILD-UPS OR RETURNS. ATC ALLOWED DEVIATIONS AT PLTS DISCRETION ACCORDING TO THE RECORDED TAPED CONVERSATION. THE PLT CONTINUED HIS FLT PATH & WASLOST ON RADAR SHORTLY THEREAFTER IN A DESCENDING RT TURN AT 16800 FT MSL. WITNESSES SAW HIM EXIT THE BOTTOM OF THE CLOUDAT ABOUT 1000 FT AGL. THE WINGS WERE SEEN TO SEPARATE FROM THE FUSELAGE. OTHER WITNESSES SAW THE FUSELAGE DESCEND INVERTED, MINUS THE WINGS, EMPANNAGE & ENGINES, TO IMPACT IN A CORN FLD. THE WRECKAGE WAS SCATTERED OVER AN AREA 1 & 1/2MILES LONG & 1/2 MILE WIDE. WITNESSES WHO HAD OBSERVED THE ACFTS PREPARATIONS FOR DEPARTURE DESCRIBED IT AS 'MASS CONFUSION' IN THE COCKPIT. THE ACFT REPORTEDLY CREPT FOWARD SIX TIMES DURING AN EXTENDED 10-15 MINUTE CHECKLIST WHILE THE ENGINES WERE RUNNING AT A HIGH RPM SETTING ON A RAMP WITH OTHER ACFT & PEOPLE AROUND.