A gear crack that led to a fatal crash of a V-22 Osprey last year may have been started by weak spots in a metal used to manufacture that part, according to documents obtained by The Associated Press.

The November crash killed eight Air Force Special Operations Command service members. It was the second time in less than two years that a catastrophic failure of a part of the Osprey’s proprotor gearbox, which serves as its transmission, caused a fatal accident. In June of 2022, five Marines were killed when a different part of the proprotor gearbox system failed.

The crashes have led to an aggressive effort by the V-22 program office and manufacturer Bell Flight to find fixes for the critical system, which has had some components wear down earlier than the military expected. While investigators still don’t know for certain what caused either crash, this latest finding might hold some clues.