WITNESSES SAW THE ACFT BRIEFLY BEFORE IT CRASHED. ONE WITNESS SAW THE ACFT FLYING TOWARD A DARK CLOUD AT AN UNKNOWN ALT, BUT TOO HIGH TO FLY UNDER IT. SHORTLY THEREAFTER, HE HEARD THE ENG NOISE INCREASE & DECREASE, THEN INCREASE UNTIL IT WAS LOUD. HE THEN HEARD WHAT SOUNDED LIKE AN EXPLOSION. HE DID NOT SEE THE CRASH, BUT SAW SMOKE FROM THE CRASH. SITE. A WOMAN CLOSE TO THE CRASH SITE SAID THE WX WAS VERY WINDY, HEAVY RAIN WAS FALLING & VISIBILITY WAS RESTRICTED. AN EXAM OF THE WRECKAGE REVEALED THE OUTBOARD WING PANELS HAD FAILED FROM POSITIVE OVERLOADS & THE LEFT HORIZONTAL STABILIZER HAD FAILED DOWNWARD. THESE PARTS WERE FOUND 3/4 TO 1 MI FROM THE MAIN WRECKAGE. PRIOR TO THE FLT, A PASSENGER, WHO PAID FOR FUEL & MINOR MAINTENANCE, SAID THE PURPOSE OF THE FLT WAS FOR SURVEYING. HOWEVER, NO SURVEYING EQUIPMENT WAS ON BOARD. SHERIFF PERSONNEL SAID THE ACFT WAS UNDER SURVEILLANCE FOR AN ILLEGAL OPERATION.