Missouri is set to execute Marcellus Williams on Tuesday in the brutal stabbing death of a former newspaper reporter in her suburban St. Louis home.

If the execution proceeds, Williams will become the third inmate executed in Missouri this year and the 15th or 16th in the nation, depending on whether he's declared dead before or after Travis James Mullis, another inmate set for execution in Texas on the same day. Two more back-to-back executions are expected in Alabama and Oklahoma on Thursday.

Williams, 55, was convicted in the Aug. 11, 1998 murder of Lisha Gayle, a former police reporter for the St. Louis Post-Dispatch described as a "once-in-a-lifetime friend" who looked for the good in people.

Williams has always maintained his innocence, a prosecutor in the case says that the execution should be called off, and no DNA has connected Williams to the crime scene. Even so, Republican Gov. Mike Parson rejected one of the last bids to halt the execution on Monday, saying in a statement that "no jury nor court, including at the trial, appellate, and Supreme Court levels, have ever found merit in Mr. Williams’ innocence claims.”

"Capital punishment cases are some of the hardest issues we have to address in the Governor’s Office, but when it comes down to it, I follow the law and trust the integrity of our judicial system," he said.

Here's what to know about Williams' case and execution.

Williams' clemency requests have been denied

Missouri Governor Mike Parson, a Republican, and the Missouri Supreme Court denied clemency to Williams on Monday, and his execution will go on as planned, according to the Innocence Project.

"Missouri is poised to execute an innocent man, an outcome that calls into question the legitimacy of the entire criminal justice system," said Williams' attorney, Tricia Rojo Bushnell, in a statement.

The St. Louis County Prosecutor tried to nullify his conviction. Still, the motion was denied after it was discovered that the trial prosecutor "contaminated potentially exculpatory DNA evidence," according to the organization.

Before the governor, the Circuit Court for St. Louis County denied the prosecuting attorney's motion to vacate Williams’ conviction and death sentence on Sept. 12.

"Mr. Williams’ case has drawn concern across the political and faith spectrum," stated the Innocence Project. "More than one million concerned citizens petitioned Governor Parson to commute Mr. Williams’ death sentence, as did a group of 69 Missouri faith leaders from the Muslim, Christian, and Jewish communities."

When is Marcellus Williams set to die by lethal injection?

Williams is slated to be executed by lethal injection after 6 p.m. Tuesday at the state prison in Bonne Terre, a city in Francois County about 60 miles southwest of St. Louis.

The method is the most common in the nation. Condemned inmates in the state may choose to die from lethal injection or lethal gas.

Williams will be administered a 5-gram dose of pentobarbital in accordance with the state of Missouri's lethal injection protocol.

The window to the death chamber in Missouri is one-way, meaning witnesses can see inmates but inmates cannot see who is watching them.

US to execute 5 men in 6 days:A look at the state of the death penalty

What was Marcellus Williams convicted of?

Gayle, who was 42, was in the shower the morning someone broke into her home on a private gated street. He entered the house through the front door after breaking a small windowpane, reaching inside, and unlocking the front door.

Wearing a long purple T-shirt, Gayle left the second-floor bathroom and was walking downstairs when she encountered the killer on the stairway landing. At some point, she was stabbed 43 times with a kitchen knife taken from the home.

Later that night, Gayle's husband found his wife's body in the couple's front foyer and called 911.

Among the evidence police collected: bloody shoeprints and fingerprints, a knife sheath and the suspect's hair collected from Gayle's shirt, hands and the floor. Missing from the house were Gayle's purse and jacket, and her husband's laptop.

Williams was convicted in 2001 and sentenced to death.

Williams' execution was previously halted twice before

Williams was previously set to be executed in January 2015 and in August 2017.

Both lethal injections were halted to conduct further DNA testing and investigation. The most recent stay of execution was ordered by then-Gov. Eric Greiten who appointed a board of inquiry to look into the case.

But during the summer of 2023, newly sworn-in Gov. Mike Parson dissolved the board and lifted the stay. The court, Parsons said, would decide Williams's fate, and the Missouri Supreme Court issued a third execution warrant for Williams.

What have Williams' defense attorneys argued?

On Aug. 21, county prosecutor Wesley Bell's office and Williams' attorneys reached an agreement allowing Williams to enter a new, no-contest plea to first-degree murder in exchange for life in prison sentence without parole. The victim's husband, Daniel Picus, signed off on the plea.

Although Bell moved to overturn Williams' murder conviction, state Attorney General Andrew Bailey argued it should stand and ordered St. Louis County Circuit Judge Bruce Hilton − who accepted the plea − to hold an evidentiary hearing on the matter.

During a Aug. 28 evidentiary hearing, the retired prosecutor who handled the case admitted evidence was mishandled in the 1998 trial that could have exonerated Williams.

But on Sept. 12, Hilton declined to vacate Williams' conviction and sentence, despite questions about DNA evidence on the knife used in the attack. In appeals, his defense produced evidence that DNA taken from the knife revealed an unknown male profile and did not match Williams.

“There is no basis for a court to find that Williams is innocent, and no court has made such a finding," St. Louis County Circuit Judge Bruce Hilton wrote. "Williams is guilty of first-degree murder, and has been sentenced to death.”

How many people are on death row in Missouri

On Monday, Williams was one of 10 people on death row in Missouri. They're all men between the ages of 44 and 68.

As of Tuesday, two men had been executed in the state this year. The most recent execution was on June 11 when the state executed David Hosier in the 2009 shooting death of Angela Gilpin, his former lover and a married mother of two.

A fourth execution is also set this year for Christopher Leroy Collings, convicted of the rape and murder of 9-year-old Rowan Ford in 2007. He is set to die by lethal injection on Dec. 3.

If Williams is put to death, it will mark the 97th time the state has executed a person.

This story was updated to add new information.

Natalie Neysa Alund is a senior reporter for USA TODAY. Reach her at nalund@usatoday.com and follow her on X @nataliealund.