Marcellus Williams, who was convicted of murder 21 years ago, has been executed in the Midwestern state of Missouri despite concerns raised over the integrity of the case.

The United States Supreme Court, the last body that could have overturned Williams’s death sentence, declined to intervene in the case on Tuesday.

The 55-year-old was executed by lethal injection shortly after 6pm (23:00 GMT) at a prison in Bonne Terre, according to The Innocence Project, whose lawyer worked with Williams. His death came a day after both Missouri Governor Mike Parson and the state’s highest court also rejected his last-ditch appeals to avoid execution.

Williams was found guilty in the 1998 killing of Lisha Gayle, a 42-year-old former newspaper reporter who was stabbed 43 times during what appeared to be a burglary gone wrong. He had maintained he was innocent.

Wesley Bell, whose office handled the original prosecution, had sought to block the execution due to concerns 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 before the execution.

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’s DNA on the murder weapon. Williams was African American.

Subsequent tests also revealed that there was 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’s lawyers to an agreement in August to commute the sentence to life in prison.

Gayle’s family also backed the deal, but Missouri Attorney General Andrew Bailey objected and the state Supreme Court blocked it at his request. A state judge upheld Williams’s murder 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’s request for clemency the same day.

“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 was among death row inmates in five states who were scheduled to be executed in the span of a week – an unusually high number amid a years-long decline in the use and support of the death penalty in the United States.

The first was carried out on Friday in South Carolina. Texas was also slated to execute a prisoner on Tuesday evening. Travis Mullis, 38, was convicted of stamping his three-month-old son, Alijah Mullis, to death in 2008.