The National Transportation Safety Board said it will begin a broad look at the railroad’s safety culture, saying it has sent investigation teams to look into five significant accidents involving Norfolk Southern since December 2021.

The agency also said it was urging the company to take immediate action to review and assess its safety practices.

Norfolk Southern didn’t respond to a request for comment. It announced plans Monday to improve the use of detectors placed along railroad tracks to spot overheating bearings and other problems in response to the derailment in Ohio last month.

NTSB has said the crew operating the train that derailed Feb. 3 outside East Palestine, Ohio, got a warning from such a detector but couldn’t stop the train before more than three dozen cars came off the tracks and caught fire.

Half of the town of about 5,000 people had to evacuate for days when responders intentionally burned toxic chemicals in some of the derailed cars to prevent an uncontrolled explosion, leaving residents with lingering health concerns. Government officials say tests haven’t found dangerous levels of chemicals in the air or water in the area.

Within the industry, Norfolk Southern has had a strong reputation for being a safe railroad over the years, said Christopher Barkan, director of the Rail Transportation and Engineering Center at the University of Illinois.

But pressure has been mounting on the railroad in the aftermath of the East Palestine derailment.

Transportation Secretary Pete Buttigieg told the nation’s freight railroads in February to immediately act to improve safety while regulators were focusing on strengthening safety rules. Buttigieg said the department will hold the railroad accountable for any safety violations that contributed to the Feb. 3 crash.

President Joe Biden said on Twitter after the derailment that the past pattern of railroads resisting safety regulations must change and that Congress should support the effort to improve safety.

Even though government data shows that derailments have declined in recent years, there were still 1,049 of them last year.

While most don’t cause any major problems, of the five accidents NTSB pointed to involving Norfolk Southern since the end of 2021, two of them resulted in the deaths of two workers.

The latest came Tuesday when a train and a dump truck collided at a steel plant in Cleveland, killing the train conductor as he stood on the outside of a car, according to authorities.