The former superintendent of Massey Energy’s Upper Big Branch Mine has pleaded guilty to felony conspiracy.Gary May was the superintendent at Upper Big Branch in April 2010, when the West Virginia mine exploded, killing 29 workers. He was charged with felony conspiracy last month, and now he has pleaded guilty.In his plea, May admitted to working with others to impede federal mine inspectors, as well as giving advance warning of inspections. He told miners to rewire a methane detector, so the equipment it was attached to could run illegally. May says he also helped hide health and safety violations, some of which may have contributed to the explosion.U.S. Attorney Booth Goodwin says mine operators have an obligation to be honest with state and federal regulators.“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,” he said. “The least we can do for coal miners is protect the integrity of the laws designed to keep them safe.”May could face up to five years in prison, and a $250,000 fine when he’s sentenced in August. So far, he is the highest-ranking mine employee to be charged in the explosion. But Goodwin says the investigation is still underway.