A Tennessee death row inmate, Byron Black, 69, cried out in pain during his execution on Tuesday after the state declined to deactivate his implanted heart defibrillator, despite concerns raised about possible suffering.

Black was executed by lethal injection at Riverbend Maximum Security Institution for the 1988 murders of his girlfriend, Angela Clay, and her two daughters, Latoya, 9, and Lakeisha, 6.

As the lethal drugs began flowing, Black appeared visibly distressed, multiple witnesses reported, according to the Daily Mail.

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He was heard sighing heavily and breathing heavily, he then repeatedly lifted his head and told his spiritual advisor, “It’s hurting so bad,” before passing away about ten minutes after the process began.

She then responded: “I’m so sorry. Just listen to my voice,” before comforting him by singing.

Legal experts say this is the first documented case where an inmate was put to death with an active defibrillator still in place, raising questions about the risk of repeated electrical shocks as his heart failed.

Black’s lawyers had long argued that deactivating the device was necessary to prevent unnecessary pain and possible violations of constitutional protections against cruel and unusual punishment.

Despite a lower court ruling in their favour, the Tennessee Supreme Court overturned the decision last week, and both the state’s governor and the US Supreme Court refused to intervene, allowing the execution to proceed.

Black’s legal team, who had sought clemency based on his intellectual disabilities as well as concerns about prolonged suffering, called the execution a violation of human rights.

Black’s longtime attorney Kelly Henry criticised the execution, saying the state “killed a gentle, kind, fragile, intellectually disabled man in a violation of the laws of our country simply because they could.”

She also maintained that “no one in a position of power, certainly not the courts, was willing to stop them” even after extensive legal arguments over whether the officials should deactivate his implantable cardioverter-defibrillator (ICD) prior to the execution.

Meanwhile, Angela Clay’s family said the execution brought some measure of closure, nearly four decades after the killings.

Black received the death penalty in 1989 for fatally shooting his girlfriend, Angela Clay, and her two daughters. According to prosecutors, he acted out of jealousy after learning that Clay might reconcile with her estranged husband, Bennie Clay, who Black had already shot the previous year.

At the time of the murders, Black was on work release from a Nashville jail, serving a two-year sentence for the earlier shooting.

After the execution, Clay’s sister said that Black would now be judged by a higher power.

“I thank God for making this happen,” Linette Bell, Angela Clay’s sister, said in a statement that was read by a victim’s advocate.

“His family is now going through the same thing we went through 37 years ago,” she added. “I can’t say I’m sorry because we never got an apology. He never apologised and he never admitted it.”