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Texas Gov. Greg Abbott has commuted the death sentence of a man who masterminded the murder of his mother and brother after the inmate's father, who barely survived the crime, pleaded for his killer son's life to be spared.

Abbott's decision, announced just a half-hour before Thomas "Bart" Whitaker was set to receive a lethal injection, is only the third time in four decades that a Texas governor has granted clemency to a death row inmate on humanitarian ground.

Whitaker, 38, will now serve a life sentence without the possibility of parole for the 2003 double murder.

Keith Hampton, Whitaker’s lawyer, said the family was in the prison’s so-called hospitality room, waiting to be taken to the death chamber to witness the execution when he called with the good news.

“I said the sentence has been commuted, and there was a lot of crying and clapping on the other end of the phone,” Hampton told NBC News.

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Abbott, who has allowed 30 executions to proceed under his watch, said he was partially swayed by the emotional appeal of the inmate's father, Kent Whitaker.

Kent Whitaker, left, is pleading for Texas Gov. Greg Abbott to grant clemency to his son Thomas before he's executed Thursday for masterminding a hit on his family. FAMILY HANDOUT / AFP - Getty Images

"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 victimized again if the state put to death his last remaining immediate family member," the governor said in a statement that also cited the unanimous recommendation of the Texas parole board.

Thomas Whitaker, 38, was convicted of hatching a 2003 plot to murder his wealthy parents and 19-year-old brother for inheritance money. His father was also shot but survived, and this week he convinced the state parole board to recommend a life sentence.

On the night of the killings, Thomas Whitaker's roommate was lying in wait with a loaded gun at the family's suburban Houston home. As they returned from a dinner out, he shot and killed Whitaker's mother, Tricia, and brother, Kevin.

In a clemency petition full of biblical quotations, Whitaker's attorneys said his deeply religious father begged the district attorney's office to seek life in prison for his son and was denied, while the actual gunman escaped a death sentence.

"Imagine two people in your family who you love most. Now, imagine one of them murders the other. There must be punishment. But would you prefer execution? What if that person was your only remaining child?" the lawyers wrote.

The parole board, they said, faced a profound question: "Is clemency warranted where execution might be justice for a wicked crime, yet would also permanently compound the suffering and grief of the remaining victim?"

Kent Whitaker embraces his wife, Tanya, after the Texas Board of Pardons and Paroles recommends clemency on Tuesday. Ralph Barrera / AP

Kent Whitaker is haunted by the murders of his wife and child, but believes that his older son has changed and that his death would be "meaningless," the petition says.

"Kent lived the assassinations. He watched his son Kevin walk into the house, heard the first and fatal shot, and saw his son's fallen body in their darkened home. He heard Tricia's last, wet coughs as Kent himself lay dying from his own gunshot wound. The bullet hit Kent nearly six inches from his heart," Whitaker's lawyers wrote.

"He also experienced the revelation that his own son was behind the killing of his beloved wife and younger son and his own attempted murder. The crucible of Kent's anguish and tribulations is beyond the ken of our imagination."

The parole board's decision Tuesday was 7-0, and no one seemed more shocked than Kent Whitaker.

"This is Texas," he said then. "This doesn't happen and I am just so encouraged that the system has worked. This was the right thing, the right thing to do."