Clayton Lockett writhed and groaned. After 43 minutes, he was declared dead

We had been warned that the execution of Clayton Lockett, a convicted murderer and rapist who had shot his 19-year-old victim and ordered a friend to bury her alive, would take longer than usual.

Jerry Massie, a spokesman for Oklahoma's corrections department, explained to the group of witnesses permitted to watch the procedure that the first drug to be used under the state's new lethal injection protocol would take some time to have its desired effect.

“Don't be surprised," he said.

In the event, the warning rang hollow. It would be a full 43 minutes after the drug was administered before Lockett died – and only after he had thrashed on the gurney, writhing and groaning – as it became clear that the procedure had been botched

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The grim scenes were the culmination of an unprecedented legal and political dispute in Oklahoma that has propelled the state into a nationwide tussle over the growing secrecy surrounding the drugs used by states to kill prisoners.

The governor of Oklahoma, Mary Fallin, had even publicly challenged the authority of a panel of judges who temporarily put a halt to the execution, in order to consider the inmates' challenges over the constitutionality of the secrecy. The court backed down, and denied the prisoners' claims. Then, in a move that attracted international attention, the state scheduled the two executions on the same night, two hours apart, in its first double execution since 1937.

It was a decision that backfired badly.

The execution of Lockett was scheduled for 6pm. A group of 12 selected media witnesses, including the Guardian, were shuttled to the white-walled Oklahoma state penitentiary in McAlester from a nearby visitor's building. We waited in the prison law library, as inmates banged on their cells and hollered to mark the event.

When officials were ready, we were taken to a viewing area, where we sat in brown metal chairs on a blue-grey carpet against the back wall. Lawyers and state officials sat in front of us. The family of Lockett's victim were in separate viewing room.The beige curtain separating the execution chamber from the viewing area was opened and the state prison warden, Anita Trammell, stood over 38-year-old Lockett. She asked him if he had any final words. He said "no."

The process began at at 6.23pm, but Lockett – as we had been warned – did not appear immediately to fall unconscious. Beneath a white sheet pulled to his neck, the restrained prisoner blinked and pursed his lips. At first he looked straight ahead, but after four minutes, he turned towards the witness area. By 6.30pm, his eyes were closed and his mouth slightly open, but when an official stood over him to check, it was clear something was wrong. "Mr Lockett is not unconscious," Trammell said.

At 6.33pm, Lockett was checked again and declared to be sedated. But then, during the following minutes, Lockett lurched forward against his restraints, writhing and attempting to speak. He strained and struggled violently, his body twisting, and his head reaching up from the gurney. Sixteen minutes after the execution began, Lockett said "Man," and Trammell decreed the blinds be lowered. Before they fell, Lockett's right arm was checked.

Then, in a gesture that seemed to echo Oklahoma’s fierce commitment to secrecy in the way it carries out lethal injections, the curtains were drawn over the execution chamber, obscuring the gruesome spectacle from public view. Officials picked up prison phones and left the room.

After a few minutes, the corrections department director, Robert Patton, came to the viewing room. "We’ve had a vein failure in which the chemicals did not make it into the offender," he told the assembled group, which included lawyers for the condemned prisoner, as well as 12 journalists.

He said the second execution – Charles Warner, who was convicted of the rape and murder of 11-month-old Adrianna Waller – would not go ahead that night.

It was unclear whether Lockett was even dead.

The witnesses left the room, and Patton then appeared before reporters gathered outside the prison, where he attempted to explain what had happened. Lockett, he said, had been administered with all three drugs in the disputed protocol. When it became clear that the drugs were not having the desired effect, the prison doctor investigated and determined that the vein into which the drugs were being administered had “blown”.

Patton said he then spoke with the prison warden and notified the state attorney general's office and governor's office that he was going to halt the execution. It was only then, 43 minutes after the process had began, that Lockett suffered what "appears to be a massive heart attack" and died inside the execution chamber, away from the eyes of witnesses.