HELENA, Mont. (AP) — Investigators probing a fatal 2021 Amtrak derailment in Montana disclosed Tuesday that the railroad track was bent along a curve near the accident site, and the problem got worse as freight trains traveled over the area before the crash.

Details on the bent track were included in hundreds of pages of investigative documents released by the National Transportation Safety Board. The three passengers who died and two who were seriously injured were in, or had just been in, an observation car that ended up on its side, the investigation found.

Railroad safety practices are facing renewed scrutiny after a fiery freight train derailment released toxic chemicals in Ohio. The Feb. 3 crash of a Norfolk Southern train forced evacuations and raised public health concerns.

Investigators identified the problem with the track based on video footage, including from two BNSF 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 get 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 investigators said.

They did not determine the final cause of the accident.

Amtrak's Empire Builder derailed Sept. 25, 2021, in northern Montana while en route from Chicago to Seattle and Portland, Oregon, with 154 people on board. The track is owned by BNSF Railway.

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

Amtrak spokesperson Marc Magliari declined comment on the track issues raised in the report. A spokesperson with BNSF Railway did not immediately respond to an email seeking comment.

Survivors described horrific scenes of people maimed and killed as four cars toppled and skidded down the tracks. Forty-four passengers and crew were taken to area hospitals with injuries.

First responders and residents of the rural plains near the crash site banded together to transport injured passengers to Chester, Montana, where they were given food and other assistance.

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.

Rail safety expert David Clarke at the University of Tennessee said a track misalignment as described in the NTSB report could "certainly" cause a derailment.

Former NTSB investigator Robert 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."

The Vardahoes were reportedly in the vestibule — the space between the observation car and a coach car behind it — when the derailment happened and were thrown from the train, investigators found. Don Vardahoe, 74, ended up under the observation car while Margie Vardahoe, 72, suffered a fatal head injury when she struck equipment along the track, a report said.

All but two of the windows on the observation car fell out during the derailment and the other fatality, Schneider, 28, and three other people were ejected from the car, investigators found.

Attorneys for the estates of the Vardahoes and Schneider did not immediately respond to a call seeking comment on Tuesday.

Schneider was dragged or rolled on the ground. Another man in the observation car suffered serious injuries and ended up with his prosthetic legs pinned under the car, the report stated. A husband and wife in the observation car fell out a window and into a gap between the side of the car and the ground as the train came to a stop. The man suffered serious injuries.

Rail accidents including derailments have been trending down in the U.S. as the number of miles traveled 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.