Officials have said that the FBI has interviewed more than 20 Massey Energy employees in a criminal investigation into the explosion that killed 29 West Virginia miners.

Additional interviews with Massey employees would be conducted within the next few weeks, officials from the federal mine safety agency and the Justice Department, speaking anonymously, told The New York Times.

Spokesmen for the federal Mine Safety and Health Administration and the FBI declined to comment.

The company was cooperating with investigators and knew of no criminal wrongdoing, Massey, owner of the Upper Big Branch Mine in Montcoal, W. Va., where the April 5 explosion occurred, said in a statement.

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. (With Inputs from Agencies)