A Missouri man was executed on Tuesday, according to the state’s Department of Corrections, even though the prosecutor’s office that secured his murder conviction 21 years ago expressed doubts about the integrity of the case and the victim’s family said he should be spared.

The US Supreme Court, the last body that could have halted the execution, declined to intervene in the case hours before Marcellus Williams, 55, was put to death by lethal injection shortly after 6pm (2300 GMT) at a prison in Bonne Terre.

His death came a day after both Missouri Governor Mike Parson and the state’s highest court also 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. He had maintained his innocence.

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

Bell said in a written statement after the execution that “if there is even the shadow of a doubt of innocence, the death penalty should never be an option.”

Tricia Rojo Bushnell, a lawyer with the Innocence Project who helped represent Williams, wrote in a statement that “the execution of an innocent person is the most extreme manifestation of Missouri’s obsession with ‘finality’ over truth, justice, and humanity, at any cost.”

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.

Williams was African American.

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.

The state Supreme Court blocked the deal at the request of Missouri Attorney General Andrew Bailey. 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.

“We hope this gives finality to a case that has languished for decades, revictimizing Ms Gayle’s family over and over again,” Parson said in a statement after the execution.

“No juror nor judge has ever found Williams’ innocence claim to be credible.”

In Williams’ petition for clemency made to the Supreme Court, his lawyers noted that Gayle’s own family thought that he should not be executed, given doubts about his guilt, and that they had approved of his life-in-prison plea deal made in August.

Laurence Komp and Laine Cardarella, with the Federal Public Defender Office in the Western District of Missouri, who also represented Williams, said in a written statement that they were baffled as to why the “admitted racial discrimination” in Williams’ trial was left unaddressed.

His public defenders said Williams adopted the name Khaliifah ibn Rayford Daniels after converting to Islam.

The corrections department released his handwritten final statement in which he said: “All praise be to Allah in every situation!!!”