Media playback is unsupported on your device Media caption Execution of Clayton Lockett (pictured): Journalist and witness Courtney Francisco describes what she saw - some may find this audio distressing.

The governor of Oklahoma has ordered an "independent review" of the state's execution protocols after a death row inmate took 40 minutes to die.

Governor Mary Fallin told reporters the execution of another inmate, due to take place the same evening, will be delayed until after the review.

Clayton Lockett's execution was stopped after 20 minutes on Tuesday evening after witnesses saw him writhing.

A ruptured vein was discovered and he died of a heart attack soon afterwards.

Ms Fallin said on Wednesday she believed in the death penalty for those who commit "heinous crimes".

"However, I also believe the state needs to be certain of its protocols and its procedures for executions and that they work."

The state's department of public safety would lead the review, she said.

Analysis Since it was first used in Texas in 1982, "the triple-drug cocktail" has become the standard execution method in US states that have the death penalty. It was designed by anaesthesiologist Stanley Deutsch as an "extremely humane" way to end life. The first drug, a barbiturate, "shuts down" the central nervous system, rendering the prisoner unconscious. The second paralyses the muscles and stops the person breathing. The third, potassium chloride, stops the heart. But critics suggest that the method may well be painful. One suggestion is that people could be too sedated by the first drug to cry out, or that they might be in pain but paralysed by the second drug. Another complication, as appears to have been the case with Clayton Lockett, is that intravenous drug use is common among death row inmates, meaning many prisoners have damaged veins that are difficult to inject. Problems sourcing some of the drugs in the official protocol have also led to claims that states are using untested drugs in their executions.

On Wednesday, a White House spokesman said the execution "fell short" of humane standards.

"[President Barack Obama] believes there are some crimes that are so heinous that the death penalty is merited," Jay Carney said.

"But it's also the case that we have a fundamental standard in this country that even when the death penalty is justified, it must be carried out humanely. And I think everyone would recognise that this case fell short of that standard."

Lockett was sentenced to death for shooting 19-year-old Stephanie Neiman and watching as two accomplices buried her alive in 1999.

Ms Neiman and a friend had interrupted the men as they robbed a home.

At his execution, he was declared unconscious and injected with the second and third drugs but three minutes later, he began breathing heavily and writhing.

Courtney Francisco, a local journalist present at the execution, told the BBC Lockett was moving his arms and legs and straining his head, mumbling "as if he was trying to talk".

Oklahoma's top prison official ordered a halt to the execution and he subsequently died.

"We believe that a vein was blown and the drugs weren't working as they were designed to." Oklahoma Department of Corrections spokesman Jerry Massie said.

The problems surrounding Lockett's execution come amid a wider debate over the legality of the three-drug method and whether its use violates guarantees in the US constitution "against cruel and unusual punishment".

'Tortured to death'

On Wednesday, the Oklahoma governor reiterated she had issued a 14-day stay of execution for fellow inmate Charles Warner, 46, who was scheduled to be put to death in the same room two hours later in a rare double execution.

But Ms Fallin said she had not given the public safety commissioner a deadline to complete his review and would continue to stay Warner's execution until it was complete.

Image copyright AFP Image caption Lockett (L) and Warner (R) were due to be put to death in the same room, just two hours apart

"His fellow Oklahomans have sentenced him to death, and we expect that sentence to be carried out as required by law," she said.‏

He was convicted of the 1997 murder and rape of an 11-month-old girl.

His lawyer, Madeline Cohen, who witnessed Lockett's execution, said Lockett had been "tortured to death".

US states have encountered increasing problems in obtaining the drugs for lethal injections, amid an embargo by European pharmaceutical firms.

Some have turned to untried combinations of drugs or have sought to obtain the drugs custom-made from compounding pharmacies. Several US states that still have the death penalty have since switched to a single-drug method.

Warner and Lockett had unsuccessfully challenged an Oklahoma state law that blocks officials from revealing - even in court - the identities of the companies supplying the drugs.

The state maintains the law is necessary to protect the suppliers from legal action and harassment.