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(KSL News) -- The family of a pilot killed in a helicopter crash four years ago says their investigation into the cause contradicts the findings of federal safety officials.
The daughter of veteran highway patrol pilot Thomas Rettberg says their private investigation shows it was mechanical failure of the chopper's drag pin assembly that caused the crash.
The pilot and his mechanic -- both of whom died -- were test-flying the chopper when it went down in a Woods Cross neighborhood just yards from some homes.
The NTSB had concluded the accident was pilot error.