One day before he’s scheduled to be executed, a Missouri death row inmate could learn whether his fate will change after the state’s Supreme Court hears arguments in his case.

Marcellus Williams, 55, was convicted of killing Felicia Gayle, a former newspaper reporter found stabbed to death in her home in 1998. Williams has long insisted he is innocent. And in an unusual move, St. Louis County’s top prosecutor filed a motion in January to vacate Williams’ 2001 conviction and sentence.

Prosecuting Attorney Wesley Bell and Williams’ lawyers on Saturday filed a joint brief asking the Missouri Supreme Court, which is hearing the case Monday, to send it back to a lower court for a “more comprehensive hearing” of the January request by Bell, a Democrat now running for Congress.

The case raises the specter of a potentially innocent person being executed – an inherent risk of capital punishment. At least 200 people sentenced to death since 1973 were later exonerated, including four in Missouri, according to the Death Penalty Information Center.

Williams is set to be executed at 6 p.m. CT Tuesday, unless the courts or Republican Gov. Michael Parson intervene.

The NAACP and the Council on American-Islamic Relations have called on Parson to halt Williams’ execution. The governor previously revoked a stay of execution in the case ordered by his predecessor, allowing plans to put Williams to death to proceed.

Why attorneys on both sides teamed up

In their joint brief, Bell and Williams’ attorneys argue the St. Louis County Circuit Court failed to credit newly disclosed evidence that contradicts representations by the prosecutor in Williams’ 2001 trial and in his prior appeals.

The Prosecuting Attorney’s Office, which handled that trial against Williams, argued in a January motion that DNA testing of the murder weapon could exclude Williams as Gayle’s killer. But the argument fell apart last month after new DNA testing revealed the murder weapon had been mishandled, contaminating the evidence meant to exonerate Williams and complicating his quest to prove his innocence.

The brief also said the court mistakenly ruled the prosecutor’s contamination of DNA evidence did not violate Williams’ due process rights. And the state attorney general’s efforts to prevent reconsideration of Williams’ conviction may have compromised proceedings in the circuit court, the brief’s authors said.

The attorneys asked the Missouri Supreme Court to vacate the circuit court’s decision and remand the case for a new hearing to give both sides time to present evidence and the court enough time to carefully consider the case.

Williams’ attorneys also asked the US Supreme Court last week to stay the execution, arguing his due process rights were denied during the yearslong legal battle to save his life.

When Parson took office, he dissolved the board and revoked Williams’ stay of execution, the petition said. The move effectively denied Williams his right to due process, Williams’ lawyers said.

They also noted former GOP Gov. Eric Greitens previously halted Williams’ execution indefinitely and formed a board to investigate his case and determine whether he should be granted clemency. “The Board investigated Williams’ case for the next six years – until Governor Michael Parson abruptly terminated the process,” the lawyers wrote.

“The Governor’s actions have violated Williams’ constitutional rights and created an exceptionally urgent need for the Court’s attention,” the court documents state.

Why a judge denied the prosecutor’s request

Prosecutors had moved to vacate Williams’ conviction because “overwhelming evidence” showed Williams’ trial had been unfair, said one of his attorneys, Tricia Rojo Bushnell.

The prosecutor’s office has raised other issues with Williams’ conviction, including claims he was convicted on the testimony of two unreliable informants facing their own legal troubles and further incentivized by $10,000 in reward money.

But ultimately, a state judge ruled against Bell’s motion to vacate Williams’ conviction and sentencing.

“There is no basis for a court to find that Williams is innocent,” Judge Bruce F. Hilton wrote in his judgment, “and no court has made such a finding. Williams is guilty of first-degree murder, and has been sentenced to death.”

The case has pitted Bell, who assumed the office in 2018, against Republican state Attorney General Andrew Bailey, who is seeking reelection. Bailey had fought Bell’s January motion, saying new DNA test results indicated the evidence would not exonerate Williams.

Last month, Bell’s office announced it had reached an agreement with Williams. Under the consent judgment approved by the court and Gayle’s family, the inmate would have entered an Alford plea of guilty to first-degree murder and be resentenced to life in prison.

But the state attorney general’s office opposed the deal and appealed to the state Supreme Court, which blocked the agreement.

CNN’s Dakin Andone and Lauren Mascarenhas contributed to this report.