"We have spoken with a representative from the (Indiana Department of Environmental Management) emergency response team on site and are awaiting additional information from them as their investigation progresses," he said.

The facility is one of two treatment plants that provides drinking water to much of the Region. The other facility is in Gary. Water from both facilities are pumped into the same system.

Until the cause has been determined, the affected areas will remain closed, the NPS said. Officials also cautioned that people should avoid eating fish from the impacted water.

IDEM and the Indiana Department of Natural Resources asked the public Thursday afternoon to avoid the east branch of the Little Calumet River in Portage while state environmental officials conduct a cleanup.

ArcellorMittal issued a statement Thursday saying dead fish were found near Burns Ditch at the southern end of the Burns Harbor facility, prompting water sampling. Preliminary results indicated cyanide and ammonia "exceedances of permitted limits," the statement read.

"We are still investigating whether these constituents are a contributing factor and what the source of these exceedances might be. We will continue to work closely with the agencies involved and provide updates as appropriate."