 Due to the lack of available evidence, no surviving crew members, no radio calls, no other witnesses and lack of a data recorder, the accident investigation board was unable to determine by clear and convincing evidence why the stabilizer trim was mispositioned.The investigation board felt that the most likely cause of this runaway stabilizer trim was a system malfunction that would have led the stabilizer trim to improperly run in a nose-down direction.The improper trim setting occurred somewhere between 14,000 and and caused a rapid and uncontrollable descent the experienced crew could not overcome.