Oklahoma's top appeals court on Thursday postponed the execution of Charles Warner by six months, while state authorities investigate the botched lethal injection of Clayton Lockett.

Lockett, 38, struggled for 43 minutes before expiring from an apparent heart attack after being injected with a lethal three-drug protocol. Execution by lethal injection usually takes 10 minutes. The state was using the sedative midazolam as part of a three-drug protocol for the first time.

Lockett was convicted of shooting and burying alive 19-year-old Stephanie Neiman and raping her friend, 18-year-old Summer Bradshaw.

Warner had been scheduled to face the death penalty on the same day as Lockett. But his execution was called off by the director of the state prison system, Robert Patton. Warner was convicted of raping and murdering his girlfriend's 11-month-old daughter.

Questions about lethal injection drugs

Meanwhile, Oklahoma Governor Mary Fallin and Attorney General Scott Pruitt have said they would not carry out any executions until an investigation into Lockett's botched lethal injection has been completed. The investigation is expected to take at least eight weeks.

"The extreme secrecy surrounding lethal injection that led to Mr. Lockett's agonizing death must be replaced with transparency in order to ensure that executions are legal and humane," Warner's attorneys, Susanna Gattoni and Seth Day, said in a press release.

Both Lockett and Warner had sued Oklahoma for refusing to disclose details about the three-drug lethal injection protocol. But the state Supreme Court dismissed the inmates' claims.

The death penalty is legal in 32 US states. According to an October 2013 Gallup poll, 60 percent of Americans support the death penalty for convicted murderers.

slk/lw (AP, AFP)