(CNN) -

A team of medical examiners has ruled that Oklahoma death row prisoner Clayton Lockett died from the state's lethal injection and not by heart attack.

The report, which was released Thursday by the Oklahoma Department of Corrections, called the manner of death a "judicially ordered execution."

The controversial April 29 execution of Lockett, who was convicted in 1999 of murder and rape, took 43 minutes from the time he was first injected.

Witnesses described the man convulsing and writhing on the gurney, as well as struggling to speak, before officials blocked the witnesses' view. The execution was halted, but Lockett eventually died.

The botched lethal injection catapulted the issue of U.S. capital punishment back into the international spotlight, raising new questions about the drugs being used and the constitutional protection against cruel and unusual punishment.

Many needle marks

The report said there were numerous attempts to start an IV because it was difficult to find "intravenous access sites."

There were at least 15 needle marks on Lockett's body, according to the autopsy, which was conducted by the Dallas-based Southwestern Institute of Forensic Sciences.

An attorney who represents a group of Oklahoma death row prisoners who have commissioned an independent autopsy said the state-sponsored exam failed to answer one vital question.

"What this initial autopsy report does not appear to answer is what went wrong during Mr. Lockett's execution," Dale Baich said, adding that more information is needed. He thinks that will be discovered in an independent autopsy.

Oklahoma Department of Corrections Director Robert Patton said in April that Lockett had died of a heart attack, but there was no support for that statement in the autopsy.

The Oklahoma Department of Public Safety said Thursday it is wrapping up an investigation that was ordered by Oklahoma Gov. Mary Fallin. The findings will be revealed next week.

New drug cocktails

Thirty-two U.S. states have the death penalty, as does the U.S. government and the U.S. military. Since 2009, three states -- New Mexico, Connecticut, and Maryland -- have voted to abolish it.

States that have capital punishment have been forced to find new drugs to use since European-based manufacturers banned U.S. prisons from using theirs for executions. One of those manufacturers is the Danish company Lundbeck, maker of pentobarbital.

Earlier this year, a convicted murderer and rapist in Ohio, Dennis McGuire, appeared to gasp and convulse for at least 10 minutes before dying from the drug cocktail used in his execution.

Lockett's execution was the first time Oklahoma had used midazolam as the first element in its three-drug cocktail. The drug is generally used for children "before medical procedures or before anesthesia for surgery to cause drowsiness, relieve anxiety and prevent any memory of the event," the U.S. National Library of Medicine said. "It works by slowing activity in the brain to allow relaxation and sleep."

Lockett was convicted in 2000 of a bevy of crimes that left Stephanie Nieman dead and two people injured.

The trouble with Lockett's execution prompted the state to delay the planned execution of Charles Warner from May to November.

In a CNN/ORC poll earlier this year, 50% of Americans said the penalty for murder in general should be death, while 45% said it should be a life sentence. The survey's sampling error made that a statistical tie. Fifty-six percent of men supported the death penalty for murder in general, while 45% of women did.

A Gallup poll last year found 62% of Americans believe the death penalty is morally acceptable, while half as many, 31%, consider it morally wrong.