UPDATE | Man dies of heart attack after botched execution in Okla.

McALESTER, Okla. (AP) — 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 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.