U.S. Attorney Booth Goodwin said May's admission shows that the obstruction of federal Mine Safety and Health Administration inspectors "was a routine matter at Upper Big Branch.''

A recent internal review by MSHA concluded that federal inspectors either missed problems or failed to examine areas where they existed in the 18 months before the blast. But the review found no evidence those failures caused it.

Last week, though, a team led by the National Institute for Occupational Safety and Health issued a report concluding that timely enforcement of existing regulations "would have lessened the chances of -- and possibly could have prevented'' the explosion.

Alpha Natural Resources of Bristol, Va., bought Massey and all its operations, including the Upper Big Branch mine, last summer.

Prosecutors have refused to say whether they are targeting former Massey CEO Don Blankenship, a hard-nosed executive whose company was cited for violations so frequently that union critics accused him of regarding fines as simply the cost of doing business.

Blankenship, once one of the most outspoken leaders in the coal industry, retired months after the explosion and moved away from West Virginia. A telephone number for him could not be found, and he has all but disappeared from public view since the blast.

"I'm glad to see he pleaded guilty, and I hope it goes up the ladder. I'm not satisfied just with him. I want other people, including Don Blankenship,'' said Jack Bowden, whose son-in-law, Steve Harrah, was among the miners who died.

Defense attorney Tim Carrico declined comment after the hearing.

Former Upper Big Branch security chief Hughie Elbert Stover was sentenced in February to three years in prison for lying to investigators and ordering a subordinate to destroy security-related documents. It's one of the stiffest punishments ever issued in a mine safety case. His attorney, William Wilmoth, is appealing the conviction.

May, 43, has cooperated with prosecutors in the continuing criminal investigation and testified at Stover's sentencing.

"That cooperation is going to be key as this investigation goes forward,'' Goodwin said.

BECKLEY, W.Va. -- The former superintendent of the Upper Big Branch Mine, where an explosion killed 29 workers, pleaded guilty Thursday to a federal fraud charge.

Gary May of Bloomingrose, the highest-ranking Massey Energy official charged in connection with the blast, faces up to five years in prison when sentenced Aug. 9.

May pleaded guilty before U.S. District Judge Irene Berger in Beckley to conspiracy to defraud the federal government. The charge stems from his actions at Upper Big Branch.

Prosecutors said May manipulated the mine ventilation system during inspections to fool safety officials and disabled a methane monitor on a cutting machine a few months before the explosion on April 5, 2010. It wasn't clear from court papers whether the device was ever fixed.

The blast was the worst U.S. mining disaster in 40 years.

Prosecutors have accused Massey of violating a host of safety laws out of a desire to put production and profits first.

Three investigations of the tragedy concluded that the company allowed highly explosive methane and coal dust to build up inside the mine, where it was ignited by a spark from an improperly maintained piece of cutting equipment. Clogged and broken water sprayers then allowed what could have been just a flare-up to become an epic blast, the investigations found.

May spoke in quiet tones and gave short answers until Berger asked him to describe what happened in relation to the charge.

"When an inspector would show up on the property, I would call up and let them know that an inspector was wanting to come underground,'' May said. "It was my intention to let them know that someone was coming.''

Berger also asked May if he acted with anyone else.

"All the station foremen, they would call up periodically, to ask if there were any inspectors,'' May replied.

U.S. Attorney Booth Goodwin said May's admission shows that the obstruction of federal Mine Safety and Health Administration inspectors "was a routine matter at Upper Big Branch.''

A recent internal review by MSHA concluded that federal inspectors either missed problems or failed to examine areas where they existed in the 18 months before the blast. But the review found no evidence those failures caused it.

Last week, though, a team led by the National Institute for Occupational Safety and Health issued a report concluding that timely enforcement of existing regulations "would have lessened the chances of -- and possibly could have prevented'' the explosion.

Alpha Natural Resources of Bristol, Va., bought Massey and all its operations, including the Upper Big Branch mine, last summer.

Prosecutors have refused to say whether they are targeting former Massey CEO Don Blankenship, a hard-nosed executive whose company was cited for violations so frequently that union critics accused him of regarding fines as simply the cost of doing business.

Blankenship, once one of the most outspoken leaders in the coal industry, retired months after the explosion and moved away from West Virginia. A telephone number for him could not be found, and he has all but disappeared from public view since the blast.

"I'm glad to see he pleaded guilty, and I hope it goes up the ladder. I'm not satisfied just with him. I want other people, including Don Blankenship,'' said Jack Bowden, whose son-in-law, Steve Harrah, was among the miners who died.

Defense attorney Tim Carrico declined comment after the hearing.

Former Upper Big Branch security chief Hughie Elbert Stover was sentenced in February to three years in prison for lying to investigators and ordering a subordinate to destroy security-related documents. It's one of the stiffest punishments ever issued in a mine safety case. His attorney, William Wilmoth, is appealing the conviction.

May, 43, has cooperated with prosecutors in the continuing criminal investigation and testified at Stover's sentencing.

"That cooperation is going to be key as this investigation goes forward,'' Goodwin said.