McALESTER, Okla. - A botched execution that used a new drug combination left an Oklahoma inmate writhing and clenching his teeth on the gurney Tuesday, leading prison officials to halt the proceedings before the inmate's eventual death from a heart attack.

Clayton Lockett, 38, was declared unconscious 10 minutes after the first of the state's new three-drug lethal-injection combination was administered. Three minutes later, he began breathing heavily, writhing, clenching his teeth, and straining to lift his head off the pillow.

The blinds were eventually lowered to prevent the viewing gallery from watching what was happening in the death chamber, and the state's top prison official eventually called a halt to the proceedings. Lockett died of a heart attack a short time later, the Department of Corrections said.

"It was a horrible thing to witness. This was totally botched," said Lockett's attorney, David Autry.

The apparent failure of the execution is likely to fuel more debate about the ability of states to administer lethal injections. That question has drawn renewed attention from death-penalty opponents in recent months, as several states scrambled to find new sources of execution drugs because drugmakers that oppose capital punishment - many based in Europe - have stopped selling to prisons and corrections departments.

Tuesday was the first time Oklahoma used the drug midazolam as the first element in its execution drug combination. Other states have used it before.

Republican Gov. Mary Fallin ordered a 14-day stay of execution for an inmate who was scheduled to die two hours after Lockett, Charles Warner.

Robert Patton, the department's director, halted Lockett's execution about 20 minutes after the first drug was administered. He later said there had been vein failure.

The execution began at 6:23 p.m., when officials began administering the first drug. A doctor declared Lockett to be unconscious at 6:33. At 6:39 - three minutes after Lockett began writhing - a doctor lifted the sheet that was covering the inmate to examine the injection site. By that time, all three drugs had been administered.

After an official lowered the blinds, Patton made a series of phone calls before calling a halt to the execution.

Lockett was declared dead at 7:06.