McALESTER, Okla. — An Oklahoma death row inmate writhed, clenched his teeth and appeared to struggle against the restraints holding him to a gurney before prison officials halted an execution in which the state was using a new drug combination for the first time. The man later died of a heart attack.

Clayton Lockett, 38, was declared unconscious 10 minutes after the first of three drugs in the state’s new lethal injection combination was administered Tuesday evening. Three minutes later, he began breathing heavily, writhing, clenching his teeth and straining to lift his head off the pillow. Officials later blamed a ruptured vein for the problems with the execution, which are likely to fuel more debate about the ability of states to administer lethal injections that meet the U.S. Constitution’s requirement they be neither cruel nor unusual punishment.

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

“It was a horrible thing to witness. This was totally botched,” said Lockett’s attorney, David Autry.

Questions about execution procedures have drawn renewed attention from defense attorneys and 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 U.S. prisons and corrections departments.

Defense attorneys have unsuccessfully challenged several states’ policies of shielding the identities of the source of their execution drugs. Missouri and Texas, like Oklahoma, have both refused to reveal their sources and both of those states have carried out executions with their new supplies.

Tuesday was the first time Oklahoma used the sedative midazolam as the first element in its execution drug combination. Other states have used it before; Florida administers 500 milligrams of midazolam as part of its three-drug combination. Oklahoma used 100 milligrams of that drug.

“They should have anticipated possible problems with an untried execution protocol,” Autry said. “Obviously the whole thing was gummed up and botched from beginning to end. Halting the execution obviously did Lockett no good.”

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. She also ordered the state’s Department of Corrections to conduct a “full review of Oklahoma’s execution procedures to determine what happened and why during this evening’s execution.”

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 midazolam. A doctor declared Lockett to be unconscious at 6:33 p.m.