(AP Photo/Jeff Gentner, File)

CHARLESTON, W.Va. - The superintendent of the West Virginia coal mine where an explosion killed 29 men was charged Wednesday with conspiracy to defraud the U.S. government, becoming the highest-ranking Massey Energy employee to face criminal prosecution so far in the worst U.S. mine disaster in four decades.

Gary May, 43, is named in a federal information, a document that signals a defendant is co-operating with prosecutors.

Reached at his home Wednesday morning, May declined comment.

U.S. Attorney Booth Goodwin said his investigation is "absolutely not" finished but did not immediately comment further.

Although other mine disasters have led to criminal charges, they've typically targeted low-ranking employees and have largely been misdemeanouroffences. A conviction on the federal fraud charge could result in fines and up to five years in prison.

It's a rare, if not unprecedented legal strategy that appears to be aimed at moving up the corporate ladder.

Last week, Goodwin urged a federal judge to make an example of the only other person charged so far, former security chief Hughie Elbert Stover. Goodwin is demanding the maximum possible sentence of 25 years in prison for actions he says contributed to the April 2010 disaster.

Stover is to be sentenced Feb. 29 for lying to federal investigators and attempting to destroy documents.

Official reports about the explosion have concluded that Virginia-based Massey Energy - which has since been bought by Alpha Natural Resources - allowed methane and coal dust to accumulate and failed to properly maintain and repair the cutting equipment that eventually created the spark that fuel needed to explode.

Clogged and broken water sprayers then allowed what could have been a minor flare-up to become an epic blast that travelled seven miles (11 kilometres) of underground corridors, doubling back on itself and killing men instantly.

The information filed in U.S. District Court in Charleston accuses May of conspiring with others to conceal many dangers in the mine through an elaborate scheme that included code words to alert miners underground when inspectors were on the property, the deliberate alteration of approved ventilation plans and the deliberate disabling of a methane gas monitor on the continuous mining machine.

Other employees of the mine have told investigators there was never enough fresh air to sweep out the highly explosive methane and coal dust that regularly accumulated - the fuel that three separate investigations have concluded powered the chain-reaction blast.

The information says Massey subsidiary Performance Coal Co. and its managers routinely violated a host of federal mine safety laws for fear that violations would cut into production time.