WASHINGTON – Federal prosecutors said Friday they are investigating whether there was “willful criminal activity” by the company that operates the West Virginia mine where 29 workers died in an accident last month.

The U.S. attorney’s office for the Southern District of West Virginia said in a letter that investigators are looking into actions by the mine’s operator, Performance Coal, and its directors, officers and agents.

The letter asks the Labor Department to hold off pursuing dozens of civil cases against Performance for alleged mine safety violations. Performance is a subsidiary of Massey Energy Co., which owns the Upper Big Branch mine.

In the letter, Assistant U.S. Attorney R. Booth Goodwin II said his office wants to make sure the pending civil cases don’t interfere with the criminal probe. He asks the Labor Department to delay 44 civil penalty cases against Performance for alleged mine safety violations.