Severe vibration caused the crash of a Bell helicopter 525 Relentless that killed two pilots during testing in 2016, according to a final NTSB report published this week.

Officials at the Fort Worth-based Bell Helicopter Textron say the company has made changes in the configuration of the aircraft’s pilot controls in response to the crash, which occurred near the Ellis County community of Italy.

“Bell and the NTSB have carefully studied the cause of the vibration, which had never been encountered before,” company officials said Wednesday in a statement. “The vibration was the result of an unanticipated combination of very high airspeed with a sustained low rotor RPM condition. The in-depth analysis of the flight data resulted in a thorough understanding of the corrective actions necessary, and appropriate changes to the aircraft have been implemented.”

The crash killed pilots Jason Grogan and Erik Boyse.

Go here to continue reading this story from the Star-Telegram