Massey Energy is being investigated by the FBI following the death of 20 miners from an explosion at its West Virginia mine.
The company is being probed for possible negligence and the bribery of state and federal inspectors, unidentified officials said.
FBI agents have reportedly interviewed more than 12 Massey employees, federal and state officials and mine union members.
Meanwhile, two law firms - Robbins Geller Rudman & Dowd LLP and Izard Nobel LLP - filed class action suits charging Massey and company officers and directors for violations of the Securities Exchange Act.
They allege Massey and some of its officers and directors violated federal securities laws by issuing misleading statements concerning the safety of its mines.
Twenty nine miners died at the Upper Big Branch mine in Montcoal, West Virginia, on April 5, in the worst US mining disaster since 1970.