PORTAGE, Ind. (CBS) — Steel manufacturer ArcelorMittal has admitted that it knew about the cause of a chemical spill that contaminated the Little Calumet River in Northwest Indiana as far back as Sunday, Aug. 11, but did not notify Indiana state officials about the subsequent spill until four days later.

ArcelorMittal told the Indiana Department of Environmental Management about the blast furnace failure that caused the spill on Thursday, Aug. 15, the IDEM said. IDEM immediately notified the news media, stakeholders, and Portage, Indiana Mayor John Cannon, and also dispatched emergency responders, the IDEM said.

CBS 2 learned earlier this week that the spill of cyanide and ammonia killed 3,000 fish.

ArcelorMittal has now apologized and accepted responsibility for the spill. They say a failure of their last furnace water recirculation system caused the release.

The Indiana Department of Environmental Management said it found a distressed fish in the river Monday, and then took complaints of a dead fish on Tuesday.

By Wednesday, the state said it discovered a significant fish die-off in the area, but they did not know the cause.

The National Parks Service temporarily banned swimming at the Portage Lakefront and Riverwalk, where contaminated water could have fed into Lake Michigan. The agency also said nobody should eat fish they catch in the area.

The state said on Thursday, ArcelorMittal told the state it had “violated the daily maximum limit for cyanide.”

Under ArcelorMittal’s permit, the company is allowed to expel a daily maximum of 0.52 mg/L of ammonia and a daily maximum of 0.0088 mg/L of cyanide. The maximum contaminant level allowed in drinking water under the Federal Safe Drinking Water Act is 0.2 mg/L, the IDEM said.

It remained unclear Tuesday afternoon how much ammonia and cyanide ArcelorMittal spilled in the most recent incident.

The latest IDEM sampling showed no cyanide in the water, and decreasing values for ammonia in-stream.

IDEM emergency crews remained at the spill site Tuesday, and the department will determine what, if any, violations occurred after emergency response activities are done.