The state of Missouri executed Marcellus Williams on Tuesday evening by lethal injection, over the objections of the local prosecutor whose office obtained Mr. Williams’s murder conviction in 2003.

Mr. Williams, who for decades maintained his innocence, had in recent days sought clemency from the governor and a stay of execution from the State Supreme Court. But on Monday, both the governor, Mike Parson, and the State Supreme Court turned him down, and on Tuesday the U.S. Supreme Court, his last hope, declined to intervene.

He was pronounced dead at 6:10 p.m. at a state prison in Bonne Terre, the Missouri Department of Corrections said in a statement.

Mr. Williams’s lawyer, Tricia Rojo Bushnell of the Midwest Innocence Project, said it was unjust to execute a man when the prosecutor’s office had admitted it was wrong and had fought to overturn the death sentence. “The execution of an innocent person is the most extreme manifestation of Missouri’s obsession with finality over truth, justice and humanity,” she said.

“Marcellus Williams should be alive today,” the local prosecutor, Wesley Bell, said in a statement. “There were multiple points in the timeline when decisions could have been made that would have spared him the death penalty.”

Over the years Mr. Williams, 55, had received stays of execution — one in 2015 and one in 2017 — but neither led to his conviction’s being thrown out.

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