A Missouri man was executed on Tuesday, according to the man's legal team, even though the prosecutor's office that secured his murder conviction 21 years ago expressed doubts about the integrity of the case.

The U.S. Supreme Court, the last body that could have halted the execution, declined to intervene in the case on Tuesday.

Marcellus Williams, 55, was put to death by lethal injection shortly after 6 p.m. 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 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, had 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 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' 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.

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.

The governor, 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 before the execution that Gayle's family opposed the killing of Williams.

"That is not justice. And we must all question any system that would allow this to occur," Bushnell wrote. "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."