ERIE, Colo. — A 2015 helicopter crash that killed two people in northern Colorado was caused by cracking in the helicopter’s rotor system. That’s according to a report released recently by the National Transportation Safety Board.

The federal investigators say that the crash near the Erie airport was caused by an in-flight break-up of the aircraft’s rotor system prompted by undetected fatigue cracking.

The crash killed flight instructor Alex Viola, a 23-year-old from Arkansas City, Kan. The crash also killed his student, 25-year-old Amy Wood of Boulder.

The Denver Post first reported the NTSB’s conclusion.

The NTSB also cited the Enstrom 280FX’s design as a contributing factor to the helicopter’s crash. The aircraft was manufactured in 1985.