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.