A bungled execution in Oklahoma “fell short of humane standards”, the White House said on Wednesday, as the state announced an investigation into how a condemned man ended up dying from a heart attack after writhing and thrashing on the gurney.

After grim scenes in the Oklahoma death chamber, where an untested cocktail of drugs failed to kill 38-year-old Clayton Lockett, lawyers representing the next set of prisoners scheduled to be executed in the US called for a moratorium on all judicial killings.

For three minutes on Tuesday night at the Oklahoma state penitentiary, Lockett groaned and writhed violently as witnesses looked on; after nearly 20 minutes the procedure was called off. Some 43 minutes after the first drugs were administered, Lockett died of what was described as a “massive heart attack”.

An autopsy was under way in Tulsa on Wednesday to determine Lockett’s manner of death. A second execution, that of another convicted killer, Charles Warner, was postponed.

The governor of Oklahoma, Mary Fallin, announced an inquiry by the head of the state's department of public safety on Wednesday. But she insisted that she was a firm supporter of the death penalty and that the execution of Warner would eventually be carried out. “Last night the state of Oklahoma executed Clayton Lockett,” she said at the opening of a press conference in Oklahoma City, striking a pointed note against widespread media coverage that portrayed the execution as botched.

In Washington, Jay Carney, the White House spokesman, said that while President Barack Obama believed the evidence showed that capital punishment was not an effective deterrent to crime, some acts were so heinous as to merit the death penalty.



But Carney said that a “fundamental standard” in the US was that executions should be carried out humanely. “I think everyone would recognise that this case fell short of that standard,” he said.

Lockett was convicted of the killing of 19-year-old Stephanie Neiman in 1999. He shot her and told another man to bury her alive. Lockett was also convicted of raping her friend in the violent home invasion that lead to Neiman's death.

Warner, 46, was found guilty of raping and killing 11-month-old Adrianna Waller in 1997. He lived with the child's mother.

Clayton Lockett, left, and and Charles Warner. Photograph: AP Photograph: AP

The executions of Lockett and Warner were scheduled two hours apart on Tuesday night after an unprecedented legal and political dispute in Oklahoma.



The inmates had challenged the secrecy surrounding Oklahoma's source of lethal injection drugs, winning at the state district court level, but two higher courts argued over which could grant a stay of execution. At one point, Fallin had publicly challenged the authority of the state supreme court when it granted a stay of execution. The court reversed its decision after the governor’s comments.



“I believe the death penalty is an appropriate response and punishment to those who commit heinous crimes against their fellow men and women,” Fallin said at the news conference. “However I also believe the state needs to be certain of its protocols and its procedures for executions and that they work.” She said that the execution of Warner, stayed until 13 May, would be postponed again if the review by the commissioner of public safety was not complete by then.

The Oklahoma attorney general, Scott Pruitt, said he will assign investigators to work with the public safety commissioner and an advisor to assess the results of the execution review.

"It is tragic that the attorney general’s office fought against transparency, but now believes that transparency is important after a botched execution," said Seth Day, an attorney for Lockett.



Dean Sanderford, one of Lockett's lawyers who witnessed the execution, said the planned review is not independent. "In order to understand exactly what went wrong in last night’s horrific execution, and restore any confidence in the execution process, the death of Clayton Lockett must be investigated by a truly independent organization, not a state employee or agency," he said.

Death penalty states have scrambled to find new execution methods after drugs companies opposed to capital punishment, mostly based in Europe, withdrew their supplies.



Oklahoma decided to lethally inject Lockett and Warner with midazolam, which acts as a sedative and is also used as an anti-seizure drug, followed by vecuronium bromide and potassium chloride. Florida has used a similar method but it employed a higher dose of midazolam. Ohio used midazolam alongside a different drug, hydromorphone, in the January execution of Dennis McGuire, which took more than 20 minutes.

The outcome on Tuesday in Oklahoma appeared likely to fuel the debate over the death penalty in the US, in particular the use of untested drug combinations.

Madeline Cohen, an attorney for Warner, condemned the way Lockett was killed. "After weeks of Oklahoma refusing to disclose basic information about the drugs for tonight's lethal injection procedures, tonight Clayton Lockett was tortured to death," she said.

Richard Dieter, the executive director of the Death Penalty Information Center, said the killing of Lockett was a “torturous action”. He said: “They did not intend for him to die in this pain – this was bungled and incompetent – but the physical result was like torture anyhow.”

Dieter said that the case was so serious that even those Americans who were in favour of the death penalty for the most serious offenders, such as Lockett, were likely to be “concerned about whether the state knows what it is doing”.

The gurney in the execution chamber at Oklahoma state penitentiary. Photograph: AP Photograph: AP

The British embassy in Washington reiterated the government's opposition to capital punishment following the botched execution of Lockett in Oklahoma.

"The UK opposes the death penalty in all circumstances as a matter of principle," a spokesman said in a statement. "Its use undermines human dignity, there is no conclusive evidence of its deterrent value, and any miscarriage of justice leading to its imposition is irreversible and irreparable. We continue to call on all countries around the world that retain the death penalty to cease its use."



Ryan Kiesel, the executive director of the American Civil Liberties Union of Oklahoma, said that by using a “science experiment” to cause Lockett to die in pain over the course of more than 40 minutes, the state had “disgraced itself before the nation and world”.



“More than any other power, the exercise of the power to kill must be accompanied by due process and transparency,” said Kiesel, adding that Lockett’s painful death had illustrated “the consequences of trading due process for political posturing”.

Mark White, a former governor of Texas and the co-chair of the Constitution Project's Death Penalty Committee, called for all US states to suspend their use of lethal injections and overhaul the execution process in order to “restore public confidence in our criminal justice system”.

Calling Lockett’s case “the horrific, if sadly predictable, result” of a “trial and error” approach to execution, White said: “Americans may want tough justice, but most do not want to be cruel or inhumane in executing even the most heinous of criminals, and this was exactly that.”

The next inmate to face the gurney in the US is Robert Campbell, 42, in Texas on 13 May.

Lawyers in that state are preparing new litigation to challenge the state’s refusal to provide any public information about where and how it is obtaining its lethal injection drugs.

Maurie Levin, one of a team of lawyers working on the new Texas litigation, said that “if nothing else, Mr Lockett’s execution in Oklahoma makes clear that you can’t simply take the word of the executioner that everything will be okay. Access to information is necessary to be able to determine whether we are at risk of an execution like what happened last night.”



Jason Clark, a spokesman for Texas department of criminal justice, said in an email to the Guardian that the execution was expected to go ahead as planned: “TDCJ has no plans to change our procedures. Texas does not use the same drugs as Oklahoma as we use a single lethal dose of pentobarbital and we have done so since 2012.”

Additional reporting by Paul Lewis in Washington and Tom Dart in Houston