Nearly two years after the Upper Big Branch coal mine explosion killed 29 workers, the mine’s former superintendent has pleaded guilty to a federal conspiracy charge.

On April 5, 2010, a blast at the Upper Big Branch mine in West Virginia caused the deadliest U.S. mine disaster since 1970. Soon after, the mine’s owner, Performance Coal Co., and its parent company, Massey Energy, fell under government scrutiny for suspected safety violations.

Former Upper Big Branch superintendent Gary May admitted yesterday to federal prosecutors that he obstructed Mine Safety and Health Administration enforcement efforts between February 2008 and April 2010 by warning mine workers about upcoming inspections and concealing violations by falsifying records, and manipulating airflow and methane monitors to fool regulators into thinking the mine was compliant. He did not, however, admit to any actions directly linked to the April 2010 explosion.

May is the highest-ranking Massey official charged in connection to the explosion. He faces up to five years in prison and a $250,000 fine, and is scheduled for sentencing on Aug. 9.

According to NPR, May’s guilty plea is a key step in the government’s efforts to criminally charge other Massey corporate executives.