The father of the man whose execution was commuted just minutes before he was scheduled to be put to death said he is grateful his son's life was spared — even though he would have been killed alongside the rest of his family if his son's plan had worked out.

Thomas 'Bart' Whitaker had been scheduled to die via lethal injection for the 2003 murders of his mum, Tricia, and brother, Kevin, in an ambush that Bart had arranged and which left his dad, Kent, shot as well.

Kent Whitaker, who has worked for years pleading for clemency for his son, had driven to the prison on Thursday with his new wife, Tanya, to say their goodbyes, reports News.com.au.

"It was extremely sad," he said. "We touched the glass with our hands and said goodbye."

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While Tanya went to the observation room during the injection, Kent decided at the last minute not to go.

Then word came that his sentence had been commuted.

Texas Governor Greg Abbott issued the rare reprieve for the 38-year-old.

"In just over three years as governor, I have allowed 30 executions. I have not granted a commutation of a death sentence until now," he said. "Mr Whitaker must spend the remainder of his life behind bars as punishment for this heinous crime."

Kent Whitaker told Megyn Kelly that he was overjoyed that his son would not be killed.

"I feel a great sense of relief and hope," he said. "He's been given a second chance at life."

Upon learning his fate, Bart told prison officials, "I'm thankful not for me but for my dad. Any punishment that I would have or will receive is just, but my dad did nothing wrong. The system worked for him today. And I will do my best to uphold my role in the system."

In commuting Bart's death sentence, Gov. Abbott said he had taken into consideration the fact that Bart's friend, Chris Brashear, had shot and killed Tricia and Kevin Whitaker and did not receive the death penalty.

He added: "Mr Whitaker's father, who survived the attempt on his life, passionately opposes the execution of his son. Mr Whitaker's father insists that he would be victimised again if the state put to death his last remaining immediate family member."

Police had originally assumed the shooting was a botched robbery, although the fact that only Bart's cell phone had been taken raised suspicions.

Bart returned to live with his wounded father for seven months before the truth emerged that it was the son who hired friend Chris Brashear to carry out the murders.

He then fled to Mexico where he lived for a year under a false named before he was arrested and extradited back to the US.

Kent, a devout Christian, said he had initially been "mad at God" after being shot in the thorax by the masked assailant who took the lives of his wife Tricia, 51, and his 19-year-old son Kevin.

"I was wrestling with my faith," he said. "But God met me in the hospital room on the night of the shootings and helped me arrive at a 'miracle' forgiveness for everyone involved," he said. "Long before I ever even suspected that that forgiveness might extend to my own son."

"I live with the extent of the loss every day and am aware of how much it has cost me — and am completely aware that all of that loss was the result of decisions made by my son," he said. "But God helped me reach that complete forgiveness and I think He did that to help me rebuild my relationship with my son."

Kent Whitaker said the prosecution's argument that Bart believed he would inherit $US1 million was wrong and that he actually had an unrecognised mental illness.

Mr Whitaker also sent letters to Gov. Abbott from other inmates showing how his son had turned his life around.

"Thomas provides care packages to newly arrived inmates — a toothbrush, razor, coffee packets, chips — because commissary privileges take a while to set up," Kent said. "He also helps other inmates with mental health illnesses."

Thomas has received a bachelor's degree in prison and is close to earning a master's degree in English literature.