Officials from the federal mine safety agency and the Justice Department, speaking anonymously, told The New York Times additional interviews with Massey employees would be conducted within the next few weeks.
Spokesmen for the federal Mine Safety and Health Administration and the FBI declined to comment.
Massey, owner of the Upper Big Branch Mine in Montcoal, W. Va., where the April 5 explosion occurred, said in a statement the company was cooperating with investigators and knew of no criminal wrongdoing.
One Upper Big Branch miner who testified before a federal grand jury said investigators seemed interested in the mine's ventilation system, the Times said.
The newspaper said the mine had been cited at least 44 times in the past two years for major methane violations.
U.S. President Barack Obama has ordered a federal investigation of the worst U.S. mine disaster in nearly four decades.