Marcellus Williams was executed Tuesday night in the US state of Missouri after spending more than two decades on death row.

Williams, who had two previous executions stayed, continued to maintain his innocence in the 1998 stabbing death of Felicia Gayle in a St. Louis suburb. Many opposed his death sentence.

An attorney representing Williams alleged racial discrimination in jury selection and mishandling of DNA evidence in the case.

Williams was denied a last-minute stay by the U.S. Supreme Court after the Missouri Supreme Court and the governor rejected his clemency requests earlier this week.

In a rare move, the U.S. Supreme Court’s three liberal justices — Sonia Sotomayor, Elena Kagan and Ketanji Brown Jackson — said Tuesday they disagreed with the conservative majority and would have granted a stay. They gave no reason.

During his trial, prosecutors said Williams broke into Mrs Gayle’s home in August 1998 and stabbed her 43 times with a large butcher knife, before stealing her purse and her husband’s laptop.

Mrs. Gayle was a social worker and former reporter for the St. Louis Post-Dispatch.

Attorneys for Williams had said there were concerns about the handling of his case, alleging that black jurors were wrongly excluded from his trial.

They also said there was no forensic evidence linking Williams to the crime scene and that the murder weapon had been misused, raising questions about DNA evidence.

The prosecutor testified that he followed procedure at the time by touching the murder weapon without gloves after it had been tested in a forensic laboratory.

Williams had requested clemency from Missouri’s Republican Governor Mike Parson, but his request was denied.

“Death penalties are among the most difficult issues we face as governor, but when push comes to shove, I will follow the law and trust the integrity of our justice system,” Parsons said in a statement Monday.

“Mr. Williams has exhausted all due process and legal remedies, including more than 15 hearings attempting to prove his innocence and overturn his conviction.”

Many people, including British billionaire Richard Branson, campaigned against the execution, the third in Missouri this year.

The victim’s family called for a life sentence instead of the death penalty, while local prosecutors pushed to overturn the conviction.

His execution was postponed twice — once in 2017 and once in 2015 — after male DNA was discovered on the murder weapon that did not match Williams’s.

Then-Governor Eric Greitens, a Republican, appointed a panel to investigate after granting a second extension. However, he resigned amid scandal and the panel never reached a conclusion.

Local District Attorney Wesley Bell also had concerns about the DNA and requested a hearing.

However, it then turned out that the DNA evidence had been falsified, because someone from the Public Prosecution Service had touched the knife without gloves. The hearing was cancelled.

The Midwest Innocence Project, a legal group whose attorneys represented Williams, attempted to reach a plea deal with the prosecution in which Williams would plead guilty to first-degree murder in exchange for a life sentence.

But the Missouri Supreme Court blocked the deal, ruling that the death penalty would stand.

Tricia Rojo Bushnell, director of the Midwest Innocence Project and Williams’ attorney, told BBC news partner CBS before the execution that the state “is on the verge of executing an innocent man, an outcome that calls into question the legitimacy of the entire criminal justice system.”