Federal prosecutors have charged the superintendent of a southern West Virginia mine with conspiracy to defraud the federal government, making him the highest-ranking Massey Energy employee yet to face criminal prosecution in a deadly explosion there. 

Gary May is named in a federal information, a document that signals he's cooperating with prosecutors.

U.S. Attorney Booth Goodwin says his investigation of the worst U.S. mine disaster in four decades isn't finished but didn't immediately comment further Wednesday.

The April 2010 blast at the Upper Big Branch mine killed 29 men.

Although other disasters have led to criminal charges, they've typically targeted low-ranking employees with misdemeanors.

The other criminal defendant is former security chief Hughie Elbert Stover. He's awaiting sentencing for lying to investigators and trying to destroy documents.