An Indiana steel plant released toxic levels of chemicals in the Little Calumet River, killing fish and shutting down parts of the Indiana Dunes Wednesday.

The Indiana Department of Environmental Management, which is leading the investigation, said a steel plant belonging to ArcelorMittal released excess amounts of cyanide and ammonia-nitrogen into the east branch of the river, causing several fish to die.

The department said it requested ArcelorMittal to help clean up the spill and monitor the chemical concentration in the Little Calumet River.

The National Park Service said it closed the water on Wednesday out to 300 feet at the Portage Lakefront and Riverwalk beach area of the Indiana Dunes National Park. Portions of the Little Calumet River were closed between Highway 149 and 249, the park service said. The trails remain open.

The waters will remain closed until the exact cause of the spill is determined and the waters are deemed safe, the park service said. Residents were told to avoid eating fish from the affected area.

A spokesperson for ArcelorMittal said in a statement that the company’s Burns Harbor steel plant was responding to the chemical release and an unrelated oil spill.

The spokesperson said the company was investigating whether the excess cyanide and ammonia in the water caused the fish to die near Burns Ditch, and what the source of the release might be.

ArcelorMittal also responded Wednesday to oil in the water at the Port of Indiana, the spokesperson said. The amount and source of the oil were known, and the spill was contained in a timely manner, the spokesperson said. The company employed a third-party spill removal service to verify that no oil had escaped the port.

“ArcelorMittal Burns Harbor is committed to environmental compliance and takes both situations very seriously,” the spokesperson said. “We will continue working cooperatively with the agencies involved on each matter to identify the responsible party and/or source and address accordingly.”