The Massey Energy-owned Upper Big Branch coal mine where 29 miners died in an explosion last month is under criminal investigation, the US Justice Department said.

The US attorney’s office for the Southern District of West Virginia said investigators are examining actions by the Massey unit, Performance Coal and its directors and officers for possible “wilful criminal activity”.

The pending civil case over safety violations is temporarily suspended until the criminal probe is completed, reports Bloomberg.

The investigation is expected to examine alleged violations of federal mine safety laws at the West Virginia mine from 2007 to 2010.

Massey is being investigated by the Federal Bureau of Investigation for possible bribery of state and federal inspectors.

Massey said in a statement on 30 April it had “no knowledge of criminal wrongdoing” and was cooperating with investigators.