AT 0938, WHILE CLBG THRU 9000', THE PLT WAS CLRD TO CLB TO FL 210. APRX 10 MIN LATER, THE ATC CTLR NOTED THE ACFT'S MODE C RTRN WASN'T BEING DISPLAYED ON HIS SCOPE. HE TRIED TO ADZ THE PLT, BUT COULDN'T ESTABLISH RADIO CONTACT. THE ACFTBROKE UP IN FLT & THE WRECKAGE WAS FND SUBMERGED IN A CREEK & ON MARSHLAND. A LARGE PIECE OF THE RGT OUTBOARD WING PANELWAS FND ABT 2 MI EAST OF THE MAIN WRECKAGE. THE RGT ENG WAS FND APRX 800' TO 1200' NE OF THE MAIN WRECKAGE IN 4' OF WTR.THERE WAS EVIDENCE THE RGT OUTBOARD WING HAD FAILED FM UPWARD & AFT OVERLOAD. NO PRE-ACDNT MECH FAILURE OR MALFUNCTION WAS FND THAT WOULD HAVE RESULTED IN AN IN-FLT BREAK-UP. RADAR DATA SHOWED THE ACFT WAS CLBG AT 115 KTS & 1100'/MIN; AT APRX 16,000' MSL, RATE OF CLB SLOWED TO APRX 750'/MIN, THEN INCREASED TO 1200'/MIN. PEAK ALT WAS APRX 18,200'. ACFT THENENTERED A STEEP DSCNT & CRASHED. AT THE APRX TIME & PLACE OF PEAK ALT, PRIMARY TARGETS APPEARED ON RADAR & REMAINED FOR SVRL MINS. ORGANIC MTRL WAS FND ON LEFT ENG INLET SCREEN, BUT SOURCE WAS NOT DTRMD. ACDNT OCCURRED ALONG BIRD FLYWAY.