For more than 20 years, Robert Roberson has been awaiting an execution for a crime he says never happened.

He was sentenced in 2003 for the death of his two-year-old daughter, Nikki, after doctors and an autopsy report concluded she died of injuries from abuse. But Roberson, his lawyers and others say she did not die from "shaken baby syndrome", as prosecutors had claimed, but complications related to pneumonia.

Prosecutors, however, insist Roberson's new evidence does not disprove their case that the child died from injuries inflicted by her father.

A diverse group is calling for clemency as Roberson's execution date of 17 October approaches. It includes 86 Texas lawmakers from both major parties, dozens of medical and scientific experts, autism advocates, lawyers, and even the lead detective in the case who helped secure Roberson’s conviction. There's also bestselling author John Grisham.

"In Robert’s case there was no crime and yet we’re about to kill somebody for it in Texas," Grisham told reporters in September.

An appeals court in 2023 agreed there was insufficient evidence to overturn the conviction. The Supreme Court declined to hear his case.

Roberson's last-ditch efforts to appeal against his conviction have failed. The Texas Board of Pardon and Paroles has until 15 October to recommend clemency, which would be up to Governor Greg Abbot to grant.

“We have to do all we can to pump the brakes before this stains Texas justice for generations,” said Democratic state representative Joe Moody.

'Unusual number of executions'

Roberson's case is the latest in a string of high-profile death row cases that have received significant public attention in recent weeks.

Maya Foa, director of Reprieve US, an anti-death penalty organisation, told the BBC that there is currently an "execution spree" in the country.

Last week, the Supreme Court heard arguments for overturning the murder conviction of Richard Glossip, found guilty of orchestrating the killing of his boss. He had been scheduled to be executed in Oklahoma nine different times.

The court will decide if his conviction should be overturned based on the allegation that prosecutors withheld information about a key witness against him, who had also lied on the stand.

The court had already put his execution on hold.

But last month, the Supreme Court declined to halt the execution of Marcellus Williams, a black man convicted of murdering a journalist in 1998.

Prosecutors had since doubted his guilt, and the victim’s family had opposed his execution. He was put to death on 24 September.

Four other men were executed the same week as Williams – making it the highest execution rate since 2003, an “unusual” number, according to Robin Maher, executive director of the Death Penalty Information Center, a non-profit research organisation that is critical of how the death penalty is applied in America.

“The unusual number of executions was not the result of any single event or co-ordinated effort,” she said.

“They simply reflected the agendas of elected state officials, who are increasingly disconnected from the interests and priorities of their constituents regarding the death penalty.”

Polls indicate support for it has declined over the past 30 years, with one recent Gallup poll suggesting 53% of Americans are in favour of capital punishment.

If Roberson is executed on Thursday, his death will be the 19th execution in 2024.

The likelihood of being executed varies widely by state. Twenty-three states do not have the death penalty, while an additional 15 have not executed anyone for at least five years. Last year five states – Texas, Oklahoma, Florida, Missouri and Alabama - accounted for all 24 executions.

'Shaken baby syndrome'

If Roberson’s death sentence is carried out, he would be the first person in the US to be executed for a “shaken baby syndrome” case.

Medical experts once used the syndrome to describe brain injuries and deaths of children who were violently shaken or assaulted. But it has come under scrutiny in recent years because of how it has been used in court cases.

In 2009, the American Academy of Pediatrics changed the name of shaken baby syndrome to "abusive head trauma". Regardless of its name, it is the leading cause of fatal brain injury in children under two.

It is usually diagnosed after finding evidence of retinal haemorrhage, brain swelling and bleeding in the brain.

While the diagnosis is broadly accepted by the medical community, a recent report highlighted the need to thoroughly examine other causes before concluding injuries were due to abuse.

"The question to be answered is, 'Is there a medical cause to explain all the findings or did this child suffer from inflicted injury?” the world's leading paediatric organisations wrote in a consensus statement published in Pediatric Radiology.

According to Roberson's account, Nikki fell out of bed before she died. He says he

comforted her and went back to sleep - but when he awoke, she wasn't breathing and her lips were blue. Roberson says he took her to hospital, where doctors said she had signs of brain death. She died the next day.

Court documents show medical staff immediately suspected abuse, because of bruises on her head, brain swelling and bleeding behind her eyes. An autopsy, conducted after Roberson was arrested, determined she died of blunt-force head trauma and her death was ruled a homicide.

Roberson’s lawyers argue that new evidence shows she had pneumonia at the time of her death that developed into sepsis.

Roberson had taken her to the hospital and to see doctors repeatedly in the days leading up to her death. His lawyers noted she was prescribed medications that are no longer given to children because they can cause serious complications. They argue the medications, plus her fall, could have accounted for the bruising, swelling and bleeding doctors found in her brain and behind her eyes.

Roberson was also diagnosed with autism after being convicted, which his lawyers say explains the lack of emotion that police witnessed when his daughter died, and biased them against him.

Brian Wharton, the lead detective in Roberson’s case who testified against him at trial, is one of the people now seeking clemency for the man.

“I will forever be haunted by the role I played in helping the state put this innocent man on death row,” he wrote in a letter of support. “Robert’s case will forever be a burden on my heart and soul.”