OMAHA, Neb. (AP) — Federal investigators on Tuesday announced a special investigation into rail operator Norfolk Southern following a fiery derailment on the Ohio-Pennsylvania border in February and several other accidents, including the death of a train conductor this week.

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 announced plans on Monday to improve the use of detectorsplaced along railroad tracks to spot overheating bearings and other problems in response to the derailment in Ohio a month ago.

The National Transportation Safety Board 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.

Norfolk Southern didn’t respond to a request for comment.