McALESTER, Okla. (AP) -- In its first lethal injection since a botched one last spring, Oklahoma executed a convicted killer with a three-drug method also used by Florida for a punishment the same night.

In an execution Thursday that lasted 18 minutes, Oklahoma inmate Charles Frederick Warner showed no physical signs of distress after the lethal drugs were administered, although he did say, "My body is on fire." He was declared dead at 7:28 p.m. CST.

Warner, 47, was put to death for killing an infant in 1997. He was originally scheduled to be executed in April on the same night as Clayton Lockett, who began writhing on the gurney, moaning and trying to lift his head after he'd been declared unconscious. That prompted state officials to impose a moratorium on executions in Oklahoma until an investigation was completed into what went wrong.

Wearing gray prison scrubs and covered with a sheet up to his waist, Warner, bald and clean shaven, was strapped to a gurney with intravenous lines in both arms.

When asked before the execution began if he had any final words, Warner responded: "Before I give my final statement, I'll tell you they poked me five times. It hurt. It feels like acid."

Warner also apologized to his family for the pain he caused them and said: "I'm not a monster. I didn't do everything they said I did."

Witnesses said they saw slight twitching in Warner's neck about three minutes after the lethal injection began. The twitching lasted about seven minutes until he stopped breathing.

Warner's attorney, Madeline Cohen, who witnessed the execution, said in a statement there was no way to know if Warner suffered because the second drug, a paralytic, would have prevented him from moving.

"Because Oklahoma injected Mr. Warner with a paralytic tonight, acting as a chemical veil, we will never know whether he experienced the intense pain of suffocation and burning that would result from injecting a conscious person with rocuronium bromide and potassium chloride," Cohen said.

It was the second time Oklahoma used the sedative midazolam as part of a three-drug method that had been challenged by Warner and other death row inmates as presenting an unconstitutional risk of pain and suffering.

The execution came after a divided U.S. Supreme Court in a 5-4 ruling said it wouldn't consider an appeal over the drugs.

In a dissenting opinion, Justice Sonia Sotomayor wrote that she believes questions about the effectiveness of the drugs are particularly important because of states' increasing reliance on 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."

Warner was executed for killing his roommate's infant daughter in Oklahoma City. Florida executed Johnny Shane Kormondy, 42, for killing a man during a 1993 home-invasion robbery in Pensacola.

Both states started their executions with midazolam. Oklahoma increased by five times the amount of the sedative it planned to use to mirror the recipe that Florida had used in nearly a dozen successful executions.

Midazolam also was used in problematic executions last year in Arizona and Ohio, however. Inmates snorted and gasped during those lethal injections that took longer than expected.

Oklahoma Attorney General Scott Pruitt has acknowledged that midazolam is not Oklahoma's first choice to be used in lethal injections. But he said prison officials have been unable to secure other, more effective drugs because the manufacturers oppose their use in executions.

After Lockett's execution was botched, a state investigation determined that a single intravenous line failed and that the drugs were administered locally instead of directly into his bloodstream.

Since then, Oklahoma has ordered new medical equipment such as backup IV lines and an ultrasound machine for finding veins, and renovated the execution chamber with new audio and video equipment to help the execution team spot potential problems.

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