HELENA, Mont. (AP) – Investigators probing a fatal 2021 Amtrak train derailment in Montana say the track had been bent along a curve near the accident site and the problem had been getting worse as freight trains traveled over it before the derailment.

Details on the bent track were included in hundreds of pages of investigative documents released Tuesday by the National Transportation Safety Board. The agency did not determine a cause.

Amtrak’s Empire Builder derailed Sept. 25, 2021 in north-central Montana, while en route from Chicago to Portland and Seattle with 154 people on board. Three people died, and dozens were injured. The track is owned by BNSF Railway.