THE AIRCRAFT DEPARTED THIEF RIVERS, MN AND PROCEEDED EN ROUTE TO NEW ULM, MN. ONE WITNESS NEAR THE ACCIDENT SITE HEARD A NOISE THAT SOUNDED TO HIM LIKE AN EXPLOSION. HE AND OTHER WITNESSES IN THE VICINITY SAW THE AIRCRAFT DESCENDING OUT OF CONTROL IN A ROLL OR A SPIN BEFORE IT CRASHED. ALSO, THEY SAW TWO OBJECTS FALLING FROM THE AIRCRAFT, ONE SMALL AND ONE LARGE. AN INVESTIGATION REVEALED THAT A PROPELLER BLADE, AND THEN THE ENGINE, HAD SEPARATED IN FLIGHT. THE PROPELLER BLADE WAS FOUND ABOUT TWO WEEKS LATER APPROXIMATELY 1/4 MILE LEFT OF THE FLIGHT PATH. AN EXAMINATION REVEALED THAT THE PROPELLER BLADE HAD FAILED FROM FATIGUE CRACKING. THE FATIGUE ORIGINATED FROM A 1/16 INCH DEEP FORGING FOLD IN THE SURFACE OF THE BLADE NEAR THE BUTT END. THE PROPELLER, A MCCAULEY MODEL 90DHB-16E, HAD 340 HRS TOTAL FLIGHT TIME.