(CNN) -- The man in charge of ensuring the safety of miners at West Virginia's Upper Big Branch coal mine pleaded guilty Thursday to charges that he helped conceal hazardous conditions from federal inspectors.
Gary May is the highest-ranking executive of former mine owner Massey Energy charged in connection with an April 2010 explosion that killed 29 miners. He could be sentenced to up to five years in prison and fined $250,000 for conspiring to defraud the federal government, the U.S. attorney's office in Charleston said.
As part of his plea agreement, May has agreed to cooperate with the investigation into the worst U.S. mining accident in decades.
"I'm pleased that Mr. May is cooperating with our investigation," U.S. Attorney Booth Goodwin said in a statement on Thursday's plea. "We hope he can give us a better picture of what was going on at this company."
In court Thursday, May admitted to tipping off mine managers that inspectors from the federal Mine Safety and Health Administration were on their way, allowing them to clean up hazards like poor airflow and piles of loose coal that would have led to citations and fines. He also admitted to falsifying safety records and telling miners to rewire a device that monitored flammable methane gas levels, allowing mine equipment to run illegally, prosecutors said.
"People who run coal mines have a fundamental obligation to be honest with mine regulators," Goodwin said. "When mine operators resort to tricks and deceit to keep government officials in the dark, our mine safety system unravels and miners are put in harm's way. The least we can do for coal miners is protect the integrity of the laws designed to keep them safe."
May faces an August 9 sentencing hearing in Beckley, West Virginia.
The Upper Big Branch explosion was the worst U.S. mine disaster since 1972, when 91 men died in a fire at the Sunshine Mine in Kellogg, Idaho. In a December report, MSHA found a methane ignition that set off flammable coal dust was the immediate cause of the 2010 explosion, but it also blamed the "unlawful policies and practices" of Massey Energy, which it said "promoted and enforced a workplace culture that valued production over safety."
Massey kept two sets of books to mislead federal inspectors and its own workers about hazards in the mine, and had twice as many accidents as it reported to regulators, the report concluded. The company failed to conduct adequate inspections, intimidated workers to prevent them from reporting violations and tipped off crews to surprise inspections, MSHA found.
Alpha Natural Resources bought Massey in 2011 and has agreed to a $209 million settlement to avoid prosecution. The deal includes payments of $1.5 million to each family that lost a member in the Upper Big Branch mine.
An earlier state investigation found the mine lacked adequate ventilation; water sprays on equipment were not properly maintained and failed to function as they should have; and the mining company didn't meet federal and state safety standards for the application of rock dust, a crucial tool in keeping highly volatile coal dust from exploding.
Massey Energy -- whose former CEO, Don Blankenship, was an outspoken critic of federal regulators -- disputed that report, saying the blast probably occurred when natural gas flooded into the mine. MSHA said that investigators considered that explanation but that it didn't fit the facts.