A Missouri man is scheduled to be executed today unless the U.S. Supreme Court intervenes at the last minute, even though the prosecutor’s office that secured his murder conviction 21 years ago has expressed doubts about the integrity of the case.

Marcellus Williams, 55, was expected to be put to death by lethal injection at 6 p.m. (1 p.m. Hawaii time) at a prison in Bonne Terre, a day after both Missouri Governor Mike Parson and the state’s highest court rejected his last-ditch bids to avoid execution.

Williams was found guilty in 2003 of killing Felicia “Lisha” Gayle, a former newspaper reporter who was stabbed to death in her home, though he has maintained his innocence.

St. Louis County prosecuting attorney Wesley Bell, whose office handled the original prosecution, has sought to block the execution due to questions about the original trial.

“Even for those who disagree on the death penalty, when there is a shadow of a doubt of any defendant’s guilt, the irreversible punishment of execution should not be an option,” Bell said in a statement.

In court papers, Bell questioned the reliability of the two main trial witnesses, concluded that prosecutors improperly excluded Black jurors on the basis of race and noted that new testing found no trace of Williams’ DNA on the murder weapon.

Subsequent tests revealed DNA on the knife from a prosecutor and an investigator who worked on the case and handled the weapon without gloves.

The contamination of the knife led prosecutors and Williams’ attorneys to reach an agreement in August calling for him to enter a no-contest plea and receive a sentence of life in prison.

But Missouri Attorney General Andrew Bailey objected, and the state Supreme Court blocked the deal at his request. A state judge upheld the conviction earlier this month, finding that the lack of evidence on the knife was not enough to establish his innocence.

The Missouri Supreme Court affirmed that decision on Monday.

Governor Parson, a Republican, also turned down Williams’ request for clemency on Monday.

“No jury nor court, including at the trial, appellate, and Supreme Court levels, have ever found merit in Mr. Williams’ innocence claims,” he said in a statement. “At the end of the day, his guilty verdict and sentence of capital punishment were upheld.”

Williams’ attorney, Tricia Rojo Bushnell of the Midwest Innocence Project, noted in a statement that Gayle’s family opposes Williams’ execution.

“Missouri is poised to execute an innocent man, an outcome that calls into question the legitimacy of the entire criminal justice system,” she said.