(CNN) -- A former coal company executive pleaded guilty Thursday and identified his old boss as part of an effort to cover up safety violations before the worst U.S. mine disaster in decades.
David Hughart, who was president of a Massey Energy subsidiary, pleaded guilty to two conspiracy counts before a federal judge in Beckley, West Virginia.
He's the highest-ranking official to admit wrongdoing in the investigation that followed the 2010 explosion at the Upper Big Branch mine in West Virginia, which killed 29 workers.
The 53-year-old Hughart has agreed to cooperate with prosecutors in the ongoing probe stemming from the disaster, the worst American mining catastrophe since 1972, according to his plea agreement. Asked during his plea hearing who he conspired with above him, Hughart replied, "The chief executive officer," CNN affilliate WCHS reported.
Massey's former CEO, Don Blankenship, was an outspoken critic of federal mine regulators, but Hughart did not name him directly.
In a statement issued Thursday afternoon, Blankenship attorney William Taylor III said Blankenship "did not conspire with anybody to do anything illegal or improper."
"To the contrary, Don took every step to make the mines under his responsibility safer," Taylor said. "We are not concerned about Mr. Hughart's recollections. People often remember untrue things when they are attempting to reduce a possible prison sentence."
Hughart faces up to six years in prison and a $350,000 fine after admitting that he and others at Massey conspired to hide safety hazards inside the company's mines between 2000 and 2010 -- a period that spans Blankenship's time at the company's helm. His sentencing is set for June 25.
His lawyer, Michael Whitt, would not comment on the statements made in court, and U.S. Attorney Booth Goodwin told CNN through a spokesman, "I'm not going to comment on the direction of the investigation."
Hughart was the president of Green Valley Resource Group, a different Massey subsidiary than the one that operated the Upper Big Branch mine. In his plea agreement, prosecutors state that he and others worked to conceal mine hazards from federal inspectors "in part because of a belief that consistently following these laws would decrease coal production."
Massey was the fourth-largest coal producer in the United States and the largest mine operator in Appalachia at the time of the Upper Big Branch explosion. But it had racked up an extensive list of violations before the disaster, and a 2011 report by the U.S. Mine Safety and Health Administration found that Massey had a history of "systematic, intentional, and aggressive efforts" to evade safety regulations.
The company kept two sets of books to mislead miners and inspectors, tipped off crews before surprise inspections and intimidated workers to prevent them from reporting violations, the report found.
Massey was bought by another company, Alpha Natural Resources, after the disaster. In December 2011, it settled with the Justice Department for a record $209 million in fines, penalties and compensation for families of the men killed at Upper Big Branch.
Another Massey manager, Gary May, admitted helping conceal hazards from inspectors in 2012. He was sentenced to 21 months in prison in January.
CNN's Matt Smith contributed to this report.