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MONETT, Mo. • The Environmental Protection Agency has begun an investigation of Tyson Foods’ role in a discharge of a food supplement that allegedly led to pure ammonia flowing into a southwest Missouri creek, killing more than 100,000 fish.

The discharge on May 16 allegedly caused the wastewater plant in Monett to fail and allowed a chemical to flow into nearby Clear Creek, The Joplin Globe reported.

The EPA did not immediately return a phone call from The Associated Press Tuesday.

The company revealed the investigation earlier this month in its quarterly notice to the U.S. Securities and Exchange Commission.

“We’re cooperating with the Environmental Protection Agency in its investigation, as we have with state and local agencies regarding this incident,” company spokesman Worth Sparkman said Monday.

The Missouri Department of Natural Resources found that Tyson’s pre-treatment plant in Monett treated wastewater containing Alimet, a liquid animal feed supplement, that it had received another Tyson operation in Aurora. After the water was pre-treated, it was discharged to Monett’s sewage system. The compound killed the bacteria that process the wastewater effluent in Monett’s plant, causing virtually undiluted ammonia to flow into Clear Creek. It is unclear how much Alimet was discharged.

Missouri Attorney General Chris Koster filed a six-count civil lawsuit against Tyson after the spill, seeking fines, compensation for damage to the stream and reimbursement for the costs of the state’s investigation.