An Oklahoma man on Thursday became the first inmate to be put to death after the state's botched execution of Clayton Lockett last year, USA Today reports. State prison officials in McAlester declared Charles Warner dead at 7:28 p.m.; it took 18 minutes to carry out the sentence.

The U.S. Supreme Court had refused to block Warner's execution over concerns about the drugs used to kill inmates in Oklahoma. In September 2014, Lockett's lethal injection was halted after about 20 minutes because of an issue with a vein. At the time, the condemned began "writhing on the gurney" and eventually died of a heart attack, according to The Associated Press. A state investigator later determined the drugs had not been administered directly into the Lockett's veins.

The state has since updated its procedures and increased the strength of midazolam, one of the drugs used in the deadly cocktail.

Before his execution, Warner complained about the treatment he had received from prison officials.

"Before I give my final statement, I'll tell you they poked me five times. It hurt. It feels like acid," he said, "I'm not a monster. I didn't do everything they said I did."

Warner had been convicted in the 1997 murder and rape of an 11-month-old girl.

"My body is on fire," Warner said after the first drug was administered despite showing no signs of distress.

Three minutes into the execution witnesses said they saw slight twitching in Warner's neck, which lasted about seven minutes until he stopped breathing.

Warner's legal team had asked the Supreme Court to halt the lethal injection after two lower federal courts refused their pleas for a stay. The body's five conservative justices denied their request without comment, but its four liberal justices issued an eight-page dissent, questioning the drug protocol.

Justice Sonia Sotomayor was critical of what she called "new and scientifically untested methods of execution."

"Petitioners have committed horrific crimes and should be punished," Sotomayor wrote. "But the Eighth Amendment guarantees that no one should be subjected to an execution that causes searing, unnecessary pain before death. I hope that our failure to act today does not portend our unwillingness to consider these questions."

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