CNN's original series "Death Row Stories" explores America's capital punishment system. Join the conversation about the death penalty at facebook.com/cnn or Twitter @CNNorigSeries using #DeathRowStories.

(CNN) A vein on an Oklahoma inmate "exploded" in the middle of his execution Tuesday, prompting authorities to abruptly halt the process and call off another execution later in the day as they try to figure out what went wrong.

The inmate, Clayton Lockett, died 43 minutes after the first injection was administered -- according to reporter Courtney Francisco of CNN affiliate KFOR who witnessed the ordeal -- of an apparent heart attack, Oklahoma Department of Corrections Director Robert Patton said.

That first drug, midazolam, is supposed to render a person unconscious. Seven minutes later, Lockett was still conscious. About 16 minutes in, after his mouth and then his head moved, he seemingly tried to get up and tried to talk, saying "man" aloud, according to the KFOR account.

Other reporters -- including Cary Aspinwall of the Tulsa World newspaper -- similarly claimed that Lockett was "still alive," having lifted his head while prison officials lowered the blinds at that time so that onlookers couldn't see what was going on.

Dean Sanderford, Lockett's attorney, said that he saw his client's body start "to twitch (and) he mumbled something." Then "the convulsing got worse, it looked like his whole upper body was trying to lift off the gurney."

Yet the office of Oklahoma Gov. Mary Fallin issued a statement indicating "execution officials said Lockett remained unconscious after the lethal injection drugs were administered."

After the ordeal, Patton told reporters that Lockett, a convicted murderer, had been sedated and then was given the second and third drugs in protocol.

"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 said, before elaborating that Lockett's vein had "exploded."

"I notified the attorney general's office, the governor's office of my intent to stop the execution and requested a stay for 14 days for the second execution scheduled this afternoon," said Patton, referring to the execution of Charles Warner.

Dianne Clay, a spokeswoman for the state attorney general's office, said Tuesday night that her office was "gathering information on what happened in order to evaluate."

The state's governor ordered an investigation and issued an executive order granting a 2-week delay in executions.

"I have asked the Department of Corrections to conduct a full review of Oklahoma's execution procedures to determine what happened and why during this evening's execution of Clayton Derrell Lockett," Fallin said in a statement.

The constitutionality of lethal injection drugs and drug cocktails has made headlines since last year, when European manufacturers -- including Denmark-based Lundbeck, which manufactures pentobarbital -- banned U.S. prisons from using their drugs in executions. Thirty-two states were left to find new drug protocols.

According to the Oklahoma Department of Corrections, its protocol includes midazolam, which causes unconsciousness, vecuronium bromide, which stops respiration, and potassium chloride, which is meant to stop the heart.

Lockett was convicted in 2000 of a bevy of crimes , including first-degree murder, first-degree rape, kidnapping and robbery in a 1999 home invasion and crime spree that left Stephanie Nieman dead and two people injured.

His final moments gave new life, at least temporarily, to Charles Warner.

Warner was convicted in 2003 for the first-degree rape and murder six years earlier of his then-girlfriend's 11-month-old daughter, Adrianna Waller.

The state decided to put off his execution set for Tuesday. But it has given no indication this delay will be indefinite despite calls from the likes of Adam Leathers, co-chair of the Oklahoma Coalition to Abolish the Death Penalty, who accused the state of having "tortured a human being in an unconstitutional experimental act of evil."

"Tonight, our state government has acted in sin and violated God's law," Leathers said. "We will pray for their souls."

Notably, Lockett and Warner -- who were both held at the Oklahoma State Penitentiary in McAlester -- had been at the center of a court fight over the drugs used in their execution.

They'd initially challenged the state Department of Corrections' unwillingness to divulge which drugs would be used, only for the department to budge and disclosed the substances.

But Lockett and Warner didn't stop there, taking issue with the state's so-called secrecy provision forbidding it from disclosing the identities of anyone involved in the execution process or suppliers of any drugs or medical equipment.

Oklahoma's high court initially issued stays on their executions, only to lift those stays last week in ruling the two men had no right to know the source of the drugs intended to kill them.

Warner's attorney, Madeline Cohen, said that further legal action can be expected given how "something went horribly awry" Tuesday.

"Oklahoma cannot carry out further executions until there's transparency in this process," Cohen said. "...I think they should all be looking at themselves hard. Oklahoma needs to take a step back."