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The head of the U.S. House of Representative’s oversight panel on Wednesday called on three drugmakers to turn over documents as part of an ongoing congressional review over generic drug price increases and accused the companies of “apparent efforts to stonewall” the probe.

U.S. House Oversight Chairman Elijah Cummings, along with U.S. Senator Bernie Sanders, the ranking member on the Senate Budget Committee, sent the letters to Mylan NV (MYL.O), Teva Pharmaceutical Industries Ltd (TEVA.TA) and privately held Heritage Pharmaceuticals, the lawmakers said in a statement.

Mylan’s shares fell 6.8% to $18.37, while Teva slumped 6.2% to $6.61 on Wednesday morning.

“Teva continues to cooperate fully with all investigations,” a spokeswoman for the company said. Mylan did not respond to Reuters request for comment.

The lawmakers first launched the probe in 2014. Earlier this year, 44 U.S. states filed a complaint in federal court alleging drug price fixing by the three pharmaceutical companies and other drugmakers, according to the statement.

Now Cummings and Sanders said they were “opening an investigation into the companies’ apparent coordinated obstruction of the investigation as revealed by,” the states’ lawsuit.

“Not only did your company’s apparent obstruction undermine our investigation, but it may have caused further harm to patients and health care providers by delaying the discovery of evidence about the companies’ price-fixing,” Cummings and Sanders wrote.

Sanders, who is among those seeking the Democratic presidential nomination for the 2020 election, has made health care and drug prices a cornerstone of his campaign.