The 55-year-old was convicted in 2003 of the murder of Lisha Gayle during an apparently botched burglary.

Marcellus Williams, who was convicted of murder 21 years ago, was executed in the midwestern state of Missouri despite doubts about the integrity of the case.

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

The 55-year-old was executed by lethal injection shortly after 6 p.m. (23:00 GMT) at a prison in Bonne Terre, reported 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 rejected his final requests to stop the execution.

Williams was found guilty in 1998 of killing Lisha Gayle, a 42-year-old former newspaper reporter who was stabbed 43 times during an apparent burglary gone wrong. He has maintained his innocence.

Wesley Bell, whose office led the original prosecution, had tried to stop the execution because of concerns about the original trial.

“Even for those who oppose the death penalty, irreversible execution should not be an option when there is even the slightest doubt about a defendant’s guilt,” Bell said in a statement before the execution.

In court documents, Bell questioned the credibility of the two main witnesses, concluded that the prosecution improperly excluded black jurors because of their race, and noted that new tests found no trace of Williams’ DNA on the murder weapon. Williams was African American.

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

Because of the knife’s contamination, prosecutors and Williams’ lawyers agreed in August to commute the sentence to life in prison.

Gayle’s family also supported the deal, but Missouri Attorney General Andrew Bailey objected and the state Supreme Court blocked it at his request. A state judge upheld Williams’ murder conviction earlier this month, saying the lack of evidence on the knife was not enough to prove his innocence.

The Missouri Supreme Court affirmed that decision on Monday.

Governor Parson, a Republican, also rejected Williams’ request for clemency that same day.

“No jury or court, at the trial, appellate, or Supreme Court level, has ever found Mr. Williams’ claims of innocence to be valid,” he said in a statement. “Ultimately, his conviction and death sentence were affirmed.”

Williams was among death row inmates in five states scheduled to be executed within a week – an unusually high number given a years-long decline in the use of and support for the death penalty in the United States.

The first execution took place on Friday in South Carolina. A prisoner was also scheduled to be executed in Texas on Tuesday evening. 38-year-old Travis Mullis was convicted of stamping his three-month-old son Alijah Mullis to death in 2008.

The death penalty has been abolished in 23 US states, and a moratorium is in place in six others – Arizona, California, Ohio, Oregon, Pennsylvania and Tennessee.