THE PLT & 6 PAX WERE RETURNING HOME AFTER FISHING IN MEXICO. AFTER CLRG CUSTOMS AT LAREDO, TX, THEY DEPARTED ON THE NEXT LEG OF THE FLT. THE PLT WAS CLRD TO 11000 FT, BUT GOT OFF COURSE. HE COMMENTED THAT AN INSTRUMENT HAD POPPED, HE THEN REQUESTED 7000 FT, BUT DUE TO OTR TRAFFIC, HIS REQUEST WAS DELAYED. WHILE STILL AT 5000 FT, HE STATED, "HEY, I'M IN THE MIDDLE OF THE TOP OF THIS STUFF." APRX 7 MIN AFTER BEING CLRD TO 7000 FT, CONTROLLERS NOTED THAT THE ACFT BGN DESCENDING. THEY ATMTD TO CALL THE PLT, BUT WERE UNABLE. AN INVESTIGATION REVEALED THE ACFT HAD CRASHED AFTER THE OUTER WING PANELS AND THE EMPENNAGE HAD SEPARATED. THE EXACT SEQUENCE OF THE IN-FLT BREAK-UP WAS NOT DETERMINED. RADAR INFO INDICATED THE ACFT HAD ENTERED SVRL VERT OSCILLATIONS OF SVRL HUNDRED FEET BFR DSCNDG. AN EXAM OF THE FRACTURE SURFACES REVEALED ONLY OVERLOAD FAILURE. OTR PLTS RPRTD CLDS BTN 3000 & 6500 FT WITH ONLY LGT TURBC. THE ACFT WAS ESTIMATED TO BE APRX 65 LBS OVR THE MAX GROSS WT LMT & LOADED NEAR THE AFT CG.