A national park in Indiana has reopened after a chemical spill earlier this month that led to death of nearly 3,000 fish in a Lake Michigan tributary.

An ArcelorMittal steel plant admitted to the spill last week, according to ABC7 News, and said that the leak was caused by a “failure at the blast furnace water recirculation system.”

The leak put toxic levels of both cyanide and ammonia-nitrogen into the Little Calumet River, ABC7 reported Thursday, causing the National Park Service to close the Portage Lakefront and parts of the Little Calumet River on August 16. The trails were kept open, however.

Portage Mayor John Cannon criticized the Indiana Department of Environmental Management, which reportedly responded to reports of “distressed fish” on August 12, but did not notify the public of the hazard until August 15.

“IDEM needs to be held accountable to let our residents and our visitors know what’s going on,” Cannon said in a statement, per ABC7, pointing out that it is peak fishing season and that the beaches are busy because of summertime.

ArcelorMittal said in a statement on August 15 that the company would “work closely with the agencies involved” to rectify the spill, according to a press release from Cannon’s office.

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IDEM and ArcelorMittal did not immediately respond to PEOPLE’s request for comment.

On Thursday, the NPS announced that the level of cyanide found in the water had reached safe levels for three consecutive days, allowing Dunes National Park, which is located on the southern shore of Lake Michigan in Porter, Indiana, to reopen.

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No cyanide was detected on two of the water testing days.

The sampling was conducted by IDEM and ArcelorMittal, but three independent labs came back with identical results, the NPS said, adding that it will “continue monitoring the ongoing water testing to ensure the health and safety of park visitors.”

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