The state of Oklahoma botched one execution and was forced to call off another on Tuesday when a disputed cocktail of drugs failed to kill a condemned prisoner who was left writhing on the gurney.

After the failure of a 20-minute attempt to execute him, Clayton Lockett was left to die of a heart attack in the execution chamber at the Oklahoma state penitentiary in McAlester. A lawyer said Lockett had effectively been "tortured to death".

For three minutes after the first drugs were delivered Lockett struggled violently, groaned and writhed, lifting his shoulders and head from the gurney.

Some 16 minutes after the execution began, and without Lockett being declared dead, the blinds separating the chamber from the viewing room were closed. The process was called off shortly afterwards. Lockett died 43 minutes after the first executions drugs were adminsitered.

The execution of Charles Warner, scheduled for 8pm local time, was then postponed. Both were due to have been carried out with a drug cocktail using dosages never before tried in American executions.

Lockett, 38, was convicted of the killing of 19-year-old, Stephanie Neiman, in 1999. She was shot and buried 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.

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 dose of midazolam five times greater. Ohio used midazolam alongside a different drug, hydromorphone, in the January execution of Dennis McGuire, which took more than 20 minutes.

The grim outcome on Tuesday in Oklahoma appeared likely to fuel the debate over the death penalty in the US, in particular the use of these untested drugs 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, which monitors capital punishment, said: "This could be a real turning point in the whole debate as people get disgusted by this sort of thing.

"This might lead to a halt in executions until states can prove they can do it without problems. Someone was killed tonight by incompetence," he told the Associated Press.

Before the attempted executions in Oklahoma, corrections spokesman Jerry Massie said they would probably take longer than normal because the first drug was expected to work more slowly. "Don't be surprised," Massie said.

The Guardian watched as Lockett was asked if he had final words. He said "no." He lay covered in a white sheet when the execution began at 6.23pm. At 6.30pm he was found to be still conscious.

Lockett was then pronounced unconscious at 6.33pm but his violent struggle began three minutes later. He tried to speak and was heard to say "man" at 6.39pm. An official in the execution room then lowered the blinds so viewers could no longer witness the process.

Robert Patton, the director of Oklahoma's department of corrections, said later that when doctors felt that the drugs were not having the required effect on Lockett, they discovered that a vein had ruptured. "After conferring with the warden, and unknown how much drugs went into him, it was my decision at that time to stop the execution," Patton told reporters.

Massie said that all three drugs in the cocktail used by the state were administered, but that a vein "blew" during the execution process and Lockett later suffered a heart attack. He was pronounced dead at 7.06pm, 43 minutes after the process began.

The execution of Charles Warner was postponed for 14 days.