 The NTSB, despite investing substantial investigative resources trying to uncover the reasons behind the two unusual and seemingly separate propeller-related malfunctions, was unable to shed light on either one.It was not able to replicate the 'pitch lock stuck' malfunction, nor adequately explain why the crew subsequently failed to effect the standard feathering procedure to properly shut down and reduce the thrust and drag on the left side.In effect, by not being able to properly secure the left engine, an unwanted asymmetric high thrust situation turned into an irreversible unwanted high asymmetric drag, which eventually resulted in an inevitable and premature descent and crash.