Death penalty opponents are speaking out after an inmate died during a botch execution in Oklahoma on Tuesday.

Clayton Lockett, 38, was supposed to die by lethal injection Tuesday night for the rape and murder of a 19-year-old woman.  However, a vein failure prevented the deadly drugs from reaching Lockett's system.  He was declared unconscious 10 minutes after receiving the injections, though he spoke on three different occasions and appeared to be in great agony, according to witnesses.

According to Courtney Francisco, a reporter for KFOR, Lockett convulsed and writhed on the gurney. He also managed to raise his head and say the words "Man," "I'm not," and "something's wrong,"

Lockett's attorney, Dean Sanderford, added that his client's body "started to twitch," and then "the convulsing got worse. It looked like his whole upper body was trying to lift off the gurney. For a minute, there was chaos," reports CNN.

Sixteen minutes into the execution, officials closed the blinds to block witnesses from seeing what was going on. State Corrections Director Robert Patton halted Lockett's execution 20 minutes after the first drug administered, which was the first time that the state administered the cocktail of injections. Eventually, Lockett died from a massive heart attack, 43 minutes later.

Now, anti-death penalty advocates and lawyers for condemned prisoners are calling for change.

"We have to stop executions until there's been a full investigation, independent investigation and full transparency," says Madeline Cohen, the lawyer who represents an Oklahoma inmate who was slated to be executed shortly after Lockett's death, reports USA Today. He received a temporary stay pending the investigation into what went wrong.

"Somebody died because of the state's incompetency," said Richard Dieter, the executive director of Death Penalty Information Center, a non-profit that advocates against capitol punishment. He added that states like Louisiana, Kentucky and Ohio will halt lethal injection until the process is better understood and there is more transparency. "I think they're going to have second thoughts and those executions will be delayed."

After administering the first of three drugs, "We began pushing the second and third drugs in the protocol," said Oklahoma Department of Corrections Director Robert Patton. "There was some concern at that time that the drugs were not having the effect. So the doctor observed the line and determined that the line had blown." He added that Lockett's vein had "exploded."

Patton also told announced that Lockett was sedated and then given the second and third drugs, before his heart gave out.