CHARLESTON, W.Va. — Former Massey Energy CEO Don Blankenship, who went to prison after a mine explosion, plans to run for the U.S. Senate seat in West Virginia held by Democratic U.S. Sen. Joe Manchin, a spokesman said Wednesday.

The formal filing was expected Wednesday, spokesman Greg Thomas told The Associated Press. The Senate clerk’s office said it hadn’t received it shortly before closing at 6 p.m.

The 67-year-old Blankenship was released from a federal prison in California in May after serving a one-year term related to the deadliest U.S. mine explosion in four decades. He is serving one year of supervised release.

Blankenship was sentenced last year for a misdemeanor conviction of conspiring to violate federal mine safety standards at Massey’s Upper Big Branch Mine in southern West Virginia, where 29 workers died in a 2010 explosion.

The U.S. Supreme Court recently rejected Blankenship’s bid to appeal. He has insisted he’s innocent, and that natural gas and not methane gas and excess coal dust caused the explosion. He has blamed Manchin for helping create the public sentiment against him. On Twitter, when he was released, Blankenship challenged Manchin to debate him.

“A U.S. Senator who says I have ‘blood on my hands’ should be man enough to face me in public,” Blankenship wrote.

At the time, Manchin responded with a statement saying he hopes Blankenship “chooses to do the right thing and disappear from the public eye.”

West Virginia GOP Chairman Conrad Lucas said he’s heard Blankenship is running. Lucas said Blankenship is a Republican.

The Republican field for next year’s election already includes West Virginia state Rep. Evan Jenkins and state Attorney General Patrick Morrisey. They have begun attacking each other, with Jenkins faulting Morrisey as an ex-lobbyist for drug companies and Morrisey criticizing Jenkins as an ex-Democrat.

Federal Election Commission records also show West Virginia native and activist Paula Jean Swearingen as a Democratic candidate for the Senate seat.