A South Carolina prison inmate convicted of killing his ex-girlfriend’s parents with a baseball bat will be the first person in the US to be executed by firing squad in 15 years.

If Brad Sigmon’s execution proceeds on Friday at 18:00 local time (23:00 GMT), three volunteers standing behind a curtain will simultaneously fire rifles at his chest with specially designed bullets.

The state’s procedure requires that those put to death by firing squad be strapped to a chair when they enter the execution chamber. The inmate then has a target placed on his heart and a bag put over his head.

Sigmon, 67, was convicted of murdering David and Gladys Larke in 2001 before kidnapping his ex-girlfriend at gunpoint. She later escaped as he shot at her.

Offered the alternatives of death by electric chair or lethal injection, Sigmon’s lawyers said he chose the more violent process because of his concerns about the effectiveness of the other two methods.

He will be the first person to be executed by firing squad in the US since 2010, and only the fourth since the country reintroduced the death penalty in 1976.

Sigmon was charged with murder in 2001 after investigators said he killed his ex-girlfriend’s parents in their home in Greenville County by alternately beating them with a bat.

He also told detectives that he planned to harm his ex-girlfriend before she escaped.

“I couldn’t have her. I wasn’t going to let anybody else have her,” he told them.

The South Carolina Supreme Court this week rejected a request from Sigmon’s lawyers to intervene. They wanted more time to learn about the drug South Carolina uses in lethal injections and questioned whether his 2002 legal representation was adequate.

That is expected to be his final appeal ahead of Friday’s planned execution.

No South Carolina governor has granted clemency to an inmate facing execution since the US legalised the death penalty again in 1976, according to the Death Penalty Information Center.

Execution by firing squad is complex.

Sigmon will be strapped in a chair with a basin built below it to catch his blood. A target will be placed on his chest and a bag over his head.

Three volunteers hidden behind a curtain will then fire at him from 15ft (4.6m) away.

The bullets used are designed to break apart on impact and cause maximum damage. Medical experts have debated the amount of pain caused by their use.

After the shots are fired, a doctor will confirm Sigmon’s death.

The state allows witnesses to observe the death from behind bulletproof glass, but the executioners will be hidden from view to protect their identities.

South Carolina passed a law in 2023 requiring that the the identities of the execution team members remain secret. It also forbids the publication of information regarding the procurement of lethal injection drugs, as a growing number of pharmaceutical companies have declined to provide them for state executions.

The American Civil Liberties Union (ACLU) filed a lawsuit challenging the state law in January.

South Carolina spent $54,000 (£41,841) constructing its firing squad area in 2022 as suppliers refused to provide prison officials with the lethal chemicals required for death by injection.

The 2023 state law passed in 2023 now shields many details about the lethal injection procedure, including the names of suppliers and the exact contents.

Most inmates sentenced to death in the state are electrocuted, but the three most recent executions were by injections that included pentobarbital. The three men were declared dead 20 minutes after being given the injection, though they appeared to stop breathing after a few minutes.

The lack of information about these executions has attracted criticism for its lack of transparency.

“This ban not only further departs from the state’s history of making execution-related information publicly available but criminalizes the disclosure of this information by anyone for any reason,” The ACLU argued in its legal complaint.

“It thus silences the scientists, doctors, journalists, former correctional officials, lawyers, and citizens who have scrutinized the safety, efficacy, morality, and legality of South Carolina’s use of lethal injection.”

Sigmon has expressed concern about the effectiveness of lethal injection.

South Carolina has released only one of two available autopsies from these deaths, which Sigmon’s lawyer say show unusual amounts of fluid in the person’s lungs.

Speaking about the decision not to die by injection, his attorney told AP: “He does not wish to inflict that pain on his family, the witnesses, or the execution team. But, given South Carolina’s unnecessary and unconscionable secrecy, Brad is choosing as best he can.”

Nationally, only three people have died by firing squad since 1976.