Corrections Director Robert Patton then postponed the execution of another inmate, Charles Warner, which was scheduled for 8 p.m.
Lockett, who was convicted of kidnapping, raping and murdering a 19-year-old woman in 1999, started convulsing after a round of lethal drug injections.
Patton told reporters that Lockett's vein had "blown" during the execution so the drugs couldn't correctly enter his body. He was reportedly trying to speak and lifted his head after doctors had declared him unconscious.
Lockett died of a heart attack at 8:06 pm.
"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,” Gov. Mary Fallin said in a statement. “I have issued an executive order delaying the execution of Charles Frederick Warner for 14 days to allow for that review to be completed.”
Lockett and Warner had unsuccessfully challenged an Oklahoma state law allowing the state to keep secret the name of companies supplying drugs used in executions.
Though the state Supreme Court ruled on April 21 that the executions of Lockett and Warner should be delayed, Fallin issued an executive order overturning the decision.
The state argued that the law protected the drug suppliers from retribution, while Lockett and Warner's lawyers said that their clients should be able to verify the quality and legality of the drugs used to execute them.
Warner's attorney, Madeline Cohen, said in a statement Tuesday that Lockett was "tortured to death."
"After weeks of Oklahoma refusing to disclose basic information about the drugs for tonight's lethal injection procedures, tonight, Clayton Lockett was tortured to death. Without question, we must get complete answers about what went wrong," she added.
"There must be an independent investigation conducted by a third-party entity, not the Department of Corrections. We also need an autopsy by an independent pathologist and full transparency about the results of its findings. Additionally, the state must disclose complete information about the drugs, including their purity, efficacy, source and the results of any testing. Until much more is known about tonight's failed experiment of an execution, no execution can be permitted in Oklahoma."
Notable deaths of 2014 [PHOTOS]