The probe examines whether companies colluded on drug pricing, according to Bloomberg. A number of companies including Mylan and Teva Pharmaceuticals have disclosed subpoenas and are cooperating with authorities, Bloomberg reported.
Shares of Mylan, Teva and Endo International plunged after the news, ending the day about 6.9 percent, 9.5 percent and 19.5 percent lower, respectively.
Lawmakers have recently turned their attention to drug pricing practices in the pharmaceutical industry, calling out price hikes at Valeant Pharmaceuticals and Mylan, among others. The key biotech ETF has fallen 27 percent in 2016, as the sector faces renewed criticism on the campaign trail.