The owners of an oil refining plant in El Dorado have agreed to pay a $4 million fine for violating federal clean air regulations.

HollyFrontier also will make improvements at its plant to reduce emissions and improve risk management practices, the Environmental Protection Agency and Kansas health officials said.

The Dallas-based company exceeded emission limits, and did not comply with chemical accident prevention and regulatory safety requirements at the El Dorado plant, which is one of the largest refineries in the Midwest.

The El Dorado refinery’s failure to prevent accidental releases of hazardous substances led to a September 2017 release a flammable hydrocarbon mixture, which caused a fire and the death of one employee, the EPA said.

The EPA said the company repeatedly violated emission standards, and some of the problems were repeat violations that were cited in a 2009 settlement involving the El Dorado refinery.