Texas inmate Robert Sparks (above), 45, who says he is intellectually disabled is set to be executed Wednesday evening for fatally stabbing his two stepsons during a 2007 attack in their North Texas home where he also killed his wife

A Texas death-row inmate who claims he is intellectually disabled is set to be executed on Wednesday evening for killing his wife and two stepsons.

Robert Sparks, 45, will receive the lethal injection at Huntsville State Penitentiary for the fatal stabbings he committed in September 2007 inside his family home in Dallas.

Prosecutors alleged that Sparks killed his wife Chare Agnew, 30, as she lay in her bed by stabbing her 18 times before he continued his murderous spree.

He then went into the bedroom of Harold Sublet, 9, and Raekwon Agnew, 10, ' bedroom and separately took them into the kitchen where he killed them too. Raekwon was stabbed at least 45 times by Sparks.

He then raped his stepdaughters - then aged 12 and 14 - after the murders.

Sparks' attorneys are asking the U.S. Supreme Court to intervene, alleging his trial jury was improperly influenced because a bailiff wore a necktie with an image of a syringe that showed his support for the death penalty.

Sparks also alleges a prosecution witness at his trial provided false testimony regarding his prison classification if a jury chose life without parole rather than a death sentence.

Sparks (above) stabbed his wife to death and then his two stepsons. After the murders, he then raped his two stepdaughters. Here he is pictured during a phone interview in 2007 - the year of the killings

He will be executed by lethal injection in Huntsville State Penitentiary on Wednesday evening (above)

Lower courts and the Texas Board of Pardons and Paroles have turned down requests by Sparks' attorneys to stop his execution.

If the execution happens, Sparks would be the 16th inmate put to death this year in the U.S. and the seventh in Texas. Seven more executions are scheduled in Texas this year.

The attorneys representing Sparks (above) have pleaded that he has an intellectual disability in the hope that it will stop the execution that is due to happen on Wednesday evening

On Tuesday, the 5th U.S. Circuit Court of Appeals declined to stop his execution on claims he's intellectually disabled, saying his attorneys had not presented sufficient evidence to show Sparks is mentally disabled and had failed to raise such a claim in a timely manner.

In August, the 5th Circuit did grant a stay for Dexter Johnson, another Texas death row inmate who also claims he is intellectually disabled. In that case, the appeals court ruled Johnson had made a sufficient showing of possible intellectual disability that needed further review.

After his arrest, Sparks told police he fatally stabbed his wife and stepsons because he believed they were trying to poison him. Sparks told a psychologist that a voice told him 'to kill them because they were trying to kill me'.

Sparks' lawyers have argued he suffers from severe mental illness and has been diagnosed as a delusion psychotic and with schizoaffective disorder, a condition characterized by hallucinations.

A psychologist hired by Sparks' attorneys said in an affidavit this month that Sparks 'meets full criteria for a diagnosis of' intellectual disability.

'Without a stay of execution, it is likely that Texas will execute an intellectually disabled man,' Seth Kretzer and Jonathan Landers, Sparks' appellate attorneys, wrote last month in court documents.

Sparks will be killed by lethal injection. Is the execution is carried out he will be the 16th inmate put to death this year in the U.S. and the seventh in Texas. (Pictured: Huntsville State Penitentiary where he will be put to death)

The Supreme Court in 2002 barred execution of mentally disabled people but has given states some discretion to decide how to determine intellectual disability. However, justices have wrestled with how much discretion to allow.

The Texas Attorney General's Office, which called the killings 'monstrous crimes,' said in court documents that Sparks' 'own trial expert testified that he was not intellectually disabled.'

His attorneys said that at the time of his trial, Sparks was not deemed intellectually disabled, but changes since then in how Texas makes such determinations and updates to the handbook used by medical professionals to diagnose mental disorders would change that.

On whether Sparks' jury was improperly influenced by the bailiff's necktie with an image of a syringe, the attorney general's office said the jury foreperson indicated she never saw the tie and had no knowledge of it affecting the jurors.

The attorney general's office said the testimony from the prosecution witness on prison classification was corrected on cross-examination.

'Sparks committed a heinous crime which resulted in the murders of two young children. He is unable to overcome the overwhelming testimony' in his case, the attorney general's office said in its court filing with the Supreme Court.