Robert Roberson’s attorney Gretchen Sims Sween speaks during a news conference in front of the Anderson County Courthouse in Palestine, after a judge denied his motion to vacate his execution at a hearing. Roberson was sentenced to death in 2003 for killing his two-year-old daughter, Nikki. (Chitose Suzuki/TNS)

HUNTSVILLE — Texas is slated to put Robert Roberson III to death Thursday evening in what could be an unprecedented execution of a disabled man convicted of killing his daughter based on scrutinized science.

Roberson, 57, was sentenced to death in 2003 for reportedly fatally shaking his 2-year-old daughter, Nikki. He is scheduled to be executed by lethal injection about 6 p.m. in Huntsville.

If he’s put to death, Roberson, who is autistic, will become the first person in the country to be executed in a “shaken baby syndrome” case, according to lawmakers.

Roberson has maintained his innocence while on death row for more than two decades.

With about three hours left until Roberson’s scheduled execution, guards set up barricades and caution tape blocking the front of the Huntsville Unit. A gaggle of TV reporters gathered on a sidewalk corner. There were no protesters, or supporters, outside the prison about 2:55 p.m.

State Rep. Brian Harrison, R-Midlothian, said lawmakers are hopeful Gov. Greg Abbott, who is set to deliver remarks on border security in Corpus Christi later Thursday, will pause the execution for 30 days.

”I spent a long time with the governor’s office last night,” Harrison told radio host Mark Davis on Thursday morning. “What I would say is very fruitful and productive discussions are happening.”

Harrison said Roberson needs a new trial because there was no direct evidence of abuse and Roberson’s own lawyer supported the shaken baby syndrome theory rather than arguing for his innocence.

Roberson’s lawyers have made a slew of 11th-hour pleas to stop his execution, arguing the prosecution hinged on “junk science.”

”This is going to be a fascinating case for the legal history books regardless of how it turns out,” Harrison said. “I now believe the criminal justice system in the state of Texas has catastrophically failed from beginning to end in this case.”

In a last-ditch effort late Wednesday, a Texas House committee subpoenaed Roberson to testify Monday at a hearing about how a state law allowing people to challenge convictions with new science was applied in his case.

It was not immediately known if the subpoena would impact Roberson’s execution.

In an email late Wednesday, a spokeswoman for the Texas Department of Criminal Justice told The Dallas Morning News that department officials had not seen the subpoena, but said if one is issued by the legislative committee, they would consult with the attorney general’s office on the “appropriate next steps.”

In addition, Roberson’s attorneys have again appealed to the U.S. Supreme Court, which has yet to rule on the matter Thursday. The nation’s highest court has previously denied their requests to intervene.

Before the House committee’s filing of the subpoena for Roberson on Wednesday, attorneys with the Texas attorney general’s office filed a brief with the U.S. Supreme Court, asking the court to reject the latest appeal from Roberson’s attorneys and arguing the execution should go forward.

The attorney general’s office has not responded to emailed inquiries since Wednesday evening. The office did not immediately return a phone call Thursday.

“It is not shocking that the criminal justice system failed Mr. Roberson so badly,” Gretchen Sween, one of Roberson’s attorneys, said in a statement sent to The News. “What’s shocking is that, so far, the system has been unable to correct itself — when Texas lawmakers recognized the problem with wrongful convictions based on discredited ‘science’ over ten years ago.”

When Roberson found Nikki unresponsive in 2002, court documents say he rushed her blue, limp body to a Palestine hospital. Roberson said she fell from a bed, but medical staff suspected child abuse and called police because of her injuries, which included bruises on her face, a bump on the back of her head and bleeding outside her brain. Her cause of death was ruled to be blunt-force head injuries.

The case against Roberson — who had just become Nikki’s sole caretaker — relied on doctors’ testimony that her death was consistent with shaken baby syndrome, when an infant is severely injured from being violently shaken.

Brain bleeds were once believed to be one in a triad of symptoms used to diagnose shaken baby syndrome. In the decades since Roberson’s conviction, research has found those symptoms are not necessarily proof of abuse.

Just last week, the Texas Court of Criminal Appeals vacated the conviction of a Dallas County man accused of injuring a child because scientific advancements undermined shaken baby syndrome. More than 30 people who served time in prison after convictions involving the theory have been declared innocent, according to the National Registry of Exonerations.

Roberson’s lawyers have said new evidence shows Nikki, who was chronically ill, died of natural and accidental causes, including “severe, undiagnosed” pneumonia. According to court documents, she had a 104.5-degree fever days before she died; her medical history included chronic infections undeterred by multiple strains of antibiotics and “alarming breathing apnea spells.”

A coalition of bipartisan state lawmakers, bestselling author and Innocence Project board member John Grisham, and a former Palestine detective whose testimony helped convict Roberson, have denounced his impending execution.

The detective, Brian Wharton, said he believes Roberson is innocent. Despite earning Roberson’s forgiveness, Wharton has said he will be “forever haunted” by participating in his arrest and prosecution.

“We rushed to judgment,” Wharton wrote in a letter to the state’s parole board. “We were wrong, the jury was misinformed, and Robert is not guilty of any crime. If we are truly a nation of laws, a people who love justice in the most meaningful sense of that word, then Robert Leslie Roberson III must be set free.”

The parole board Wednesday did not recommend clemency to Roberson, who had petitioned for his death sentence be commuted or the execution delayed 180 days.

An Anderson County judge Tuesday rejected arguments from Roberson’s attorneys that his death warrant is illegitimate because the judge who oversaw post-conviction proceedings erred.

The state’s highest criminal appeals court has also repeatedly refused to intervene with Roberson’s death sentence.

Roberson would be the sixth person put to death this year in Texas, which has led the nation in executions since the death penalty was reinstated in 1976. Eight people were executed in the state in 2023.

Staff writers Nolan D. McCaskill and Aarón Torres in Austin and Chase Rogers in Dallas contributed to this report.