HELENA, Mont. (AP) — Investigators probing a 2021 fatal Amtrak derailment in Montana disclosed Tuesday that the track had been bent along a curve near the accident site and that the problem had been getting worse as freight trains traveled over the area before the derailment that killed three people and injured dozens of others.

Details on the bent track were included in hundreds of pages of investigative documents released 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 Seattle and Portland, Oregon, with 154 people on board. The track is owned by BNSF Railway.

Survivors described horrific scenes of people maimed and killed as four cars — including an observation car with large windows — toppled and skidded down the tracks.

Killed in the accident were Margie and Don Vardahoe, a Georgia couple on a cross-country trip to mark their 50th wedding anniversary, and Zachariah Schneider, 28, a software developer from Illinois. Forty-four passengers and crew were taken to area hospitals with injuries.

The train was traveling just below the speed limit of 79 mph (127 kph) when its emergency brakes were activated, the NTSB has said.

The conclusion that the track was bent came from video footage, including from two freight trains that went around the accident curve within 90 minutes before the Amtrak derailment.

After each of the freight trains had passed, the bend in the track appeared to have gotten worse, investigators reported. Neither of the freight train crews saw the problem.

“The Amtrak engineer also did not see the track misalignment before traveling over that portion of track. Investigators, who watched the videos and were focused primarily on detecting the track deviation, were able to first see the misalignment only a couple seconds before the trains had traveled over it,” the report said.

Former NTSB investigator Robert J. Chipkevich, who had not reviewed the findings in the 2021 crash, said track problems can be caused by issues including the quality of maintenance and repairs, the integrity of the ballast material that underlies the rails and excessive summer heat.

“These conditions can change certainly after a heavy freight train passes over it,” Chipkevich said. “If it wasn't noticed by a crew member, then you have several tons of train crossing over that so it could change.”

Amtrak spokesperson Marc Magliari declined comment on the track issues raised in the report.

Railroad safety practices are facing renewed scrutiny following a fiery Ohio freight train derailment involving toxic chemicals. The Feb. 3 accident forced evacuations and raised public health concerns.

Rail accidents including derailments have been trending down in the U.S. as the number of miles travelled by trains drops.

However, the rate of accidents per mile has been going up, according to the Federal Railroad Administration. Railway unions contend rail transportation has become riskier in recent years following widespread job cuts. Most rail accidents involve freight trains, and fatalities involving passenger trains are rare.