“The purpose of the investigation was to identify the cause and contributing factors that led to this mishap,” Lt. Gen. Tony Bauernfeind, then the commander of the Air Force Special Operations Command, said.

“By conducting a thorough review and accident and safety investigation, we hope to provide answers to the families of the airmen that lost their lives and prevent future occurrences and tragedies.”

Th crash caused concerns about the safety of the Osprey, which takes off and lands like a helicopter but flies like an airplane. It has been used on numerous missions involving U.S. special forces.

“The board president found, by a preponderance of the evidence, the mishap was caused by a catastrophic failure of the left-hand prop rotor gear box that created a rapidly cascading failure of the aircraft’s drive system, resulting in an instantaneous asymmetric lift condition that was unrecoverable by the mishap crew,” Air Force Special Operations Command said.

“Additionally, the board president found decision-making was casual, prolonging the mishap sequence and removing any consideration of an earlier landing at a different divert location.”

The command said inadequate risk management and ineffective crew resource management were also contributing factors to the crash.

Tim Loranger, an attorney representing families of the airmen who died in the crash, said in a statement to NBC News that the gear box failure could not have been mitigated by any amount of skill or experience from the crew.

“The loss of these brave individuals is a profound tragedy and any suggestion that places the blame on those who are no longer here to defend their actions only adds to the deep pain their families are already enduring,” Loranger said.

The use of the Osprey was halted in the United States and Japan after the crash off the shore of Yakushima in Japan, but the Pentagon gave the green light for the aircraft to fly again in March after its investigation.