Death row inmate makes plea for mercy claiming autism caused him to be wrongly jailed for killing two-year-old daughter

A father who has spent more than 20 years on death row for the murder of his two-year-old daughter has begged for his conviction to be overturned while claiming that his undiagnosed autism landed him in prison.

Robert Roberson, 57, was sentenced to death for killing his young child Nikki Curtis at his home in Palestine, East Texas, in 2003 after a series of tragic events which unfolded the previous year.

He had taken the little girl to hospital on 31 January, 2002, and told medics that she had fallen from her bed after a period of being unwell - with hospital staff noticing that he appeared to have a lack of emotion towards his gravely injured child.

Nikki had serious head injuries as well as bruising on her chin, cheek and jaw and a subdural haematoma, with one nurse describing the child as being 'limp and blue'.

Suspicious of her seemingly uncaring father and the nature of the tot's injuries, staff at the hospital alerted the police.

Robert Roberson maintains that he did not kill his little girl Nikki (KLTV)

Roberson was subsequently arrested after his daughter passed away and was put on trial for her murder, with prosecutors saying that the dad had caused Nikki's death by violently shaking her, rather than her falling off a bed.

He was found guilty by a jury and it was ruled he would be executed, with investigators concluding that the two-year-old had been a victim of the now largely discredited shaken baby syndrome (SBS).

Now, the Death Row inmate is making an eleventh hour plea to the justice system, as a lot has changed in the more than two decades that have passed since - including Roberson being diagnosed with autism.

With support from organisations such as the Innocence Project, the prisoner is appealing to get his conviction overturned while pointing to the fact that doctors now refrain from using SBS as a cause of death.

Even the scientist who first proposed the idea of SBS confessed that the term had become exploited and was being used 'to put innocent people in prison', while warning in 2012 that 'we have gone badly off the rails'.

Children's charity NSPCC said that SBS 'is outdated and doesn’t cover the other causes of non-accidental head injury in infants and young children', adding that there 'is a stigma of violence and blame around the term'.

The 57-year-old has been on Death Row since 2003 (Texas Department of Criminal Justice)

As well as this, Roberson's so-called deadpan expression and demeanour which aroused suspicion in the hospital is also being looked at through a new lens after he was diagnosed with autism during his time in prison.

The dad's inability to explain his daughter's condition also raised red flags for medics, who took it as a sign he was lying.

Speaking to The Guardian last year, Roberson claimed he had trained himself not to cry in childhood as he would 'get his behind whooped' by his father if he saw him getting upset.

"I would never think about shaking her," he added. And that's the God's honest truth. I don't know what happened to her.

"I hope and pray that God gives them the knowledge for the people to make a righteous decision. I know I didn’t do it. I’m not guilty. So I’m at peace with the Lord."

Nikki had been unwell in the week prior to her death and had been taken to the local emergency room, where she was prescribed Phenergan - which now comes with a warning about being prescribed to young kids - and sent home.

As her condition didn't improve, she was taken back to the health centre where doctors prescribed her further Phenergan and codeine - which is an opioid now restricted for children under the age of 18.

Roberson was diagnosed with autism during his time in prison (KLTV)

The two-year-old had gone to sleep beside Roberson the following night, before he then woke up the next day and realised she was unconscious.

Even a retired detective who worked the case in 2002 and testified against the dad during trial has said that Roberson's proposed execution would be 'a tragic, irreversible mistake'.

Brian Wharton headed to the hospital after receiving reports that a mortally ill child had been brought in and arrested the death row inmate just hours after her death - without any autopsy results.

Roberson had a prior criminal record for burglary and theft, and had violated his parole before his daughter's death.

Last week, in an op-ed published by the Dallas Morning News, Wharton said he believes that 'something went very wrong' in Roberson's case and that he 'fears justice was not served'.

The former cop wrote: "If there is no movement to correct this injustice, I fear myself and others will carry our guilt eternally. I have come to believe that Nikki died of accidental and natural causes.

"I am convinced that she was not murdered. Roberson is innocent. There was no crime. I believe this because the 'science' that was used to obtain Roberson's arrest and conviction has changed drastically since his arrest."

If you’ve been affected by any of these issues and want to speak to someone in confidence regarding the welfare of a child, contact the NSPCC on 0808 800 5000, 10am-8pm Monday to Friday. If you are a child seeking advice and support, call Childline for free on 0800 1111, 24/7.