The day after Oklahoma inmate Clayton Lockett died of a heart attack following a botched execution, one of his attorneys spoke to The Washington Post and described what he saw.

“It was complete horror,” Dean Sanderford, a public defender, said Wednesday. “It was the most awful thing I’ve ever seen.”

Nothing seemed to happen at the beginning, Sanderford said. Eventually, Lockett’s eyes closed as the drug appeared to put him to sleep. Minutes later, it was announced that Lockett was unconscious.

“But then, after that, he started twitching, started mumbling,” Sanderford said. “I couldn’t understand what he was saying. And it just started to get worse. It looked like, to me, someone waking up from anesthesia, except it looked far more terrible. There was convulsing, his body kept lifting up off the gurney like he was trying to sit up. His eyes opened at one point, he started mumbling more. It was pretty clear to me that he was coming back into consciousness.”

That was when the curtain was lowered. Not long after, the execution was called off.

“We were just freaked out,” he said. “It was an awful thing to watch.”

Lockett’s family didn’t witness his death because because he had urged them not to come, Sanderford said.

“I think he suffered an excruciatingly painful death, and I think it was prolonged,” he said. “This is exactly the kind of execution everyone was afraid of with this combination of drugs. … I was horrified. And it wasn’t just me. Even the officials, they were horror-struck. They’d never seen anything like this either.”

Sanderford said there needs to be an investigation, autopsy and report from someone who is not a state doctor. In addition, Oklahoma needs to reveal where they obtained the drugs used in the execution.

“They don’t need to be carrying out any more executions until they come clean, until we know exactly what happened with Clayton’s execution and everything about these drugs, where they’re getting them,” he said.