THE PLT DEPARTED LONGMONT, CO AT ABOUT 0520 MDT. AT 0617 MDT, HE CONTACTED THE DENVER EN ROUTE FLT ADVISORY SERVICE & RECEIVED WX INFO. THE LOCATION OF THE ACFT AT THAT TIME WAS NOT DETERMINED. NO FURTHER RADIO COMMUNICATIONS WERE RECEIVED FROM THE PLT. A SEARCH WAS INITIATED WHEN THE ACFT DID NOT ARRIVE AT ITS DESTINATION. IT WAS FOUND ON 9/9/83 WHERE IT IMPACTED IN MOUNTAINOUS TERRAIN AT AN ELEVATION OF ABOUT 12,500 FT. IMPACT OCCURRED ON STEEP, ROCKY TERRAIN. AN EXAM OF THE WRECKAGE REVEALED THE ACFT HAD CRASHED IN A RIGHT WING LOW, NOSE DOWN ATTITUDE WHILE AT A HIGH RATE OF SPEED. NO EVIDENCE OF A PREIMPACT/MECHANICAL MALFUNCTION OR FAILURE WAS FOUND. THERE WERE NO KNOWN WITNESSES. HOWEVER, PERSONNEL IN THE VICINITY STATED THAT THE SKY WAS OVERCAST, CLOUD BASES WERE AT ABOUT 10,000 FT, THE MOUNTAINS WERE OBSCURED & LIGHT DRIZZLE WAS FALLING.