THE ACFT WAS ON A X-COUNTRY FLT FROM CAMARILLO, CA TO OSHKOSH, WI. THE PLT LANDED AT ABOUT NOON AT MONTROSE, CO, WHERE THE ACFT WAS REFUELED & THE OCCUPANTS WENT TO A RESTAURANT TO EAT. AFTER DEPARTING MONTROSE, THE ACFT WAS SEEN FLYING LOW OVER RISING, MOUNTAINOUS TERRAIN, HEADING TOWARD WILLIAMS PASS. SUBSEQUENTLY, IT COLLIDED WITH TREES & CRASHED ON THE SOUTH SLOPE OF A VALLEY NEAR WILLIAMS PASS, APRX 92 MI ENE OF MONTROSE. THE WITNESSES DID NOT SEE OR HEAR THE ACFT CRASH. AT THE TIME, THEY THOUGHT THE PLT MAY HAVE TURNED OR SOMEHOW CONTINUED OVER THE PASS. LATER, THE ACFT WAS FOUND AFTER A WITNESS LEARNED THERE WAS A MISSING PLANE & NOTIFIED AUTHORITIES. NO EVIDENCE OF A PREIMPACT, MECHANICAL MALFUNCTION OR FAILURE WAS FOUND. THE DENSITY ALTITUDE WAS ESTIMATED TO BE 14,400 FT. THE PLT'S LAST BIENNIAL FLT REVIEW WAS ON 7/24/81.