A steelmaker in Indiana is denying manipulating results from toxic readings that were required after a chemical spill killed more than 3,000 fish last year.

The Post-Tribune reports state regulators claim that ArcelorMittal’s steel plant is redoing daily ammonia and cyanide tests, reporting the lower score.

An Indiana Department of Environmental Management official says the company’s attempt to redo daily tests “undermines the integrity” of its self-disclosures.

ArcelorMittal denies all allegations. The steelmaker was required to do daily testing after the spill of cyanide and ammonia led to the fish kill and prompted closures of beaches along Lake Michigan.