A MAINT SUPERVISOR & 2 PLTS NOTED THAT THE L ENG DID NOT RESPOND NORMALLY WHILE BEING STARTED. AFT STARTING, THE PLT TAXIED TO RWY 10 FOR AN IFR DEPARTURE. SINCE THE TWY DID NOT EXTEND TO THE END OF THE RWY, THE PLT OF ANOTHER ACFT WAITED ON THE TWY, JUST SHORT OF THE RWY, APRX 2967 FT FROM THE END. HE OBSD SMOKE COMING FROM THE L ENG OF THE MU-2 & REPORTED THIS INFO ON THE UNICOM FREQ. HE REPORTED THAT THE MU-2 CONTD & ROTATED FOR TKOF APRX 200 TO 250 YDS EAST OF HIS PSN. RPRTDLY, THE ACFT CLIMBED TO APRX 300-400 FT AFL, THEN TURNED LEFT TWD HIGHER TERRAIN. AFTER TURNING, IT ENTER WHAT 2 WITNESSES DESCRIBED AS A VMC ROLL, THEN CRASHED & BURNED. AN EXAM OF THE AIRFRAME REVEALED NO PREIMPACT MECHANICAL FAILURES. A TEARDOWN OF THE L ENG REVEALED THAT A TURBINE BEARING, PN 3101092-1, HAD BEEN MISALIGNED & SUBSEQUENTLY BEGAN TO FAIL, DAMAGING THE 2ND & 3RD STAGES OF THE TURBINE SECTION. THE ARPT ELEV WAS 6597 FT. THE COMPUTED TKOF ROLL WAS 1620 FT & THE COMPUTED ACCELERATE-STOP DISTANCE WAS 3100 FT. THE LNDG GEAR & SEL, FND IN DWN PSN.