The U.S. Justice Department is investigating the country’s four biggest meat processors for antitrust violations, according to a report in Bloomberg News.

The department’s antitrust division has issued “civil investigative demands,” the equivalent of subpoenas, to Tyson Foods, JBS SA, Cargill Foods and National Beef Inc. The four companies collectively control some 80% of the U.S. beef market.

The coronavirus pandemic and consequent meat plant closures have highlighted the concentration in the American meat processing industry. Last month, a group of attorneys general from several states wrote a letter to U.S. Attorney General Bill Barr complaining that cattle growers are being squeezed with low prices, while consumers have to pay higher prices at retail. Bloomberg reports that the U.S. Justice Department is communicating with several state attorneys general about coordinated action.

The Justice Department’s probe follows criminal charges of price-fixing against the CEO of Pilgrim’s Pride, which is majority-owned by JBS, and others.