WASHINGTON (Reuters) – Former Massey Energy Co Chief Executive Officer Donald Blankenship’s bid to overturn his conviction stemming from a 2010 West Virginia mine explosion that killed 29 coal miners ended on Tuesday as the U.S. Supreme Court rejected his appeal.

Blankenship was released from prison in May after serving a one-year sentence for conspiring to violate federal mine safety standards. He had asked the Supreme Court to hear his challenge to a January ruling by a federal appeals court in Richmond, Virginia upholding his conviction.

Blankenship had argued that the trial judge improperly instructed jurors about the meaning of “willfully” violating mine safety regulations.

Once dubbed West Virginia’s “king of coal” for his working-class background and tough approach to business, Blankenship helped build Massey into Appalachia’s largest coal producer, with more than 7,000 employees and more than 40 mines.

Blankenship failed to convince a lower court that his December 2015 conviction on the misdemeanor conspiracy charge should be voided because the federal indictment did not specify which U.S. mine safety regulations he conspired to violate, and because of several errors by the trial judge.

He said jurors were allowed to convict him if they found he disregarded the requirements of safety regulations, but willfulness requires proof that he actually knew that his conduct violated the law. Blankenship argued that the high court should intervene in his case because prosecutors sought to blame him for the tragedy in a rush to judgment.

“It was permeated throughout with unchecked abuses of power by prosecutors intent on securing a conviction by any means possible,” Blankenship’s lawyers told the high court in a filing.

Blankenship was also fined $250,000 in connection with his conviction. He was acquitted on related felony charges.

The fire caused by a methane or natural gas leak likely set off the April 2010 blast at Massey’s now-closed Upper Big Branch mine, located about 40 miles (65 km) south of Charleston, according to federal investigators. The death toll was the highest in a U.S. mine accident since 91 workers died in a 1972 Idaho silver mine fire.

In May, Blankenship wrote a letter to President Donald Trump urging Trump to “get to the truth” about the incident.