A man on death row in South Carolina, previously contemplating execution by firing squad, chose lethal injection instead on Friday after witnessing another prisoner struggle to die when bullets seemingly missed the heart.

Stephen Stanko was advised by his legal team that lethal injection might simulate the sensation of drowning, as pentobarbital floods the veins, causing fluid to surge into the lungs.

State-appointed medical experts claim the drugs induce unconsciousness before any pain is experienced, while inmate-hired specialists argue the fluid rush might mimic drowning sensations.

The U.S. Supreme Court denied a delay to his June 13 execution Wednesday that was requested so he could learn more about his options. 

Mikal Mahdi was executed in April.  (South Carolina Department of Corrections via AP)

Stanko had been sentenced to death twice for two separate murders. 

In April 2006, Stanko, 57, beat and strangled girlfriend Laura Ling to death and raped her teenage daughter and slit her throat. The daughter survived and testified against him at his trial. 

Hours after the murder, he went to his 74-year-old friend Henry Turner’s home, shot him to death and stole his truck. 

Defense attorney Gerald Kelly confers with defendant Stephen Stanko during a pretrial hearing at the Georgetown County Courthouse in Georgetown, S.C., July 31, 2006.    (AP Photo/Tom Murray, Pool, File)

South Carolina resumed executions in September after a 13-year break after it ran out of lethal injection drugs and pharmacies refused to provide more unless a new secrecy law protected their privacy. 

The Associated Press contributed to this report.