Death row inmate Clayton Lockett died after writhing in agony for 40 minutes in botched execution that lawyer described as 'torture'

A death row inmate in Oklahoma died of a heart attack after spending 43 minutes writhing in agony on a stretcher because of a botched execution that one lawyer has described as "torture".

Clayton Lockett, 38, who was sentenced to death for shooting a 19-year-old woman and then watching his friends bury her alive, was administered a sedative and declared unconscious by a doctor in accordance with prison execution protocol.

Just minutes later, however, he began breathing heavily, writhing, clenching his teeth and straining to lift his head off the pillow. Lockett, who was due to be executed along with fellow inmate Charles Warner, apparently experienced a vein failure which prevented the experimental cocktail of drugs, which have never been used in Oklahoma before, from being fully effective.

The blinds were subsequently lowered to prevent those in the viewing gallery from watching what was happening in the death chamber, and the state's senior prison official eventually called a halt to the proceedings. Lockett died of a heart attack a short time later, the Department of Corrections said.

"It was a horrible thing to witness. This was totally botched," said Lockett's lawyer, David Autry.

Fellow inmate Charles Warner's execution was subsequently delayed and will now take place in 14 days time.

Mr Warner's lawyer Madeline Cohen condemned the prison authorities for refusing to reveal basic information about what drugs would be used in the execution and said Mr Lockett had been "tortured to death".

Warner, 46, was convicted of the 1997 rape and murder of 11-month-old Adrianna Walker, the daughter of his roommate.

Republican Governor Mary Fallin ordered a 14-day stay of execution for the inmate who was scheduled to die two hours after Lockett. She also ordered the state's Department of Corrections to conduct a "full review of Oklahoma's execution procedures to determine what happened and why during this evening's execution".

The botched execution came to light after tweets from AP reporter Bailey Elise McBride who was present at the controversial double execution.

It is likely to once again raise questions over the ability of states to administer lethal injections that meet the US Constitution's requirement they be neither cruel nor unusual punishment.

Yesterday was the first time Oklahoma used the drug midazolam as the first element in its execution drug combination.

Other states have used it before - Florida administers 500 milligrams of midazolam as part of its three-drug combination. Oklahoma used 100 milligrams of that drug.

"They should have anticipated possible problems with an untried execution protocol," Mr Autry said.

"Obviously the whole thing was gummed up and botched from beginning to end. Halting the execution obviously did Lockett no good."

Mr Autry was sceptical about claims that Mr Lockett had suffered compromised veins. "I'm not a medical professional, but Mr Lockett was not someone who had compromised veins," he said. "He was in very good shape. He had large arms and very prominent veins."

In Ohio, the January execution of an inmate who made snorting and gasping sounds led to a civil rights lawsuit by his family and calls for a moratorium.

The state has stood by the execution but said on Monday that it is boosting the dosages of its lethal injection drugs.

Timeline of the botched execution.

• 18:23 - Sedative administered
• 18:33 - Doctor declares Lockett unconscious
• 18:36 - Lockett is restless. Doctor discovers a ruptured vein
• Curtain drawn
• Execution halted
• 19:06 - Lockett dies reportedly from a heart attack

Additional reporting by AP