CNN's original series "Death Row Stories" explores America's capital punishment system Sundays at 10 p.m. ET/PT on CNN. Join the conversation about the death penalty at facebook.com/cnn or Twitter @CNNorigSeries using #DeathRowStories.
(CNN) -- The execution of Clayton Lockett was halted after a doctor acknowledged that because of problems in administering the drugs, there were not enough left to carry out the death sentence, the Oklahoma Department of Corrections said Thursday.
The revelation came as Oklahoma's governor called for an investigation into the attempted execution that, according to some witnesses, left Lockett writhing in pain after a vein collapsed. A new and previously unused drug combination was reportedly used in the attempted execution.
A timeline released by the state Department of Corrections details what occurred after Lockett's execution was abruptly halted and a shade was lowered, blocking the view of witnesses.
"The doctor checked the IV and reported the blood vein had collapsed, and the drugs had either absorbed into tissue, leaked out or both," according to the timeline.
The director of the corrections department then asked whether Lockett had been given enough of the drug combination to kill him, and the doctor said "no."
"Is another vein available? And if so, are there enough drugs remaining?" the doctor was asked, according to the timeline.
The doctor's answer to both questions: "No."
Lockett's attempted execution was carried out Tuesday at the Oklahoma State Penitentiary in McAlester, where he had been housed following his conviction and death sentence for shooting Stephanie Nieman and then watching as two others buried her alive in 1999.
Lockett died from an apparent heart attack, 43 minutes after the execution began.
The autopsy on Lockett, being conducted at Southwestern Institute for Forensic Science in Dallas, will take up to 12 weeks, Capt. George Brown of the Oklahoma Department of Prisons said. He did not offer an explanation for the length of time for the autopsy.
According to the Oklahoma Department of Corrections, its execution protocol includes midazolam, which causes unconsciousness, vecuronium bromide, which stops respiration, and potassium chloride, which is meant to stop the heart.
What follows is the Department of Corrections timeline of the execution:
-- At 5:22 p.m. local time, Lockett is restrained on the execution table.
-- Between 5:27 p.m. and 6:18 p.m., he is examined by a phlebotomist, who examines his arms, legs and feet for appropriate veins to administer drugs.
-- At 6:18 p.m., an IV is inserted into a vein in Lockett's groin area, and his lower body is then covered with a sheet.
-- At 6:23 p.m., the curtain in the execution chamber is raised, and the doctor administers the first drug intravenously.
-- At 6:30 p.m., Lockett is still conscious.
-- At 6:33 p.m., the doctor declares Lockett is unconscious and begins to administer the second and third drugs.
-- At 6:42 p.m., the curtain is lowered.
-- At 6:44 p.m., the doctor checks the IV and reports Lockett's vein has collapsed.
-- Between 6:44 p.m. and 6:56 p.m., the prison warden calls Department of Corrections Director Robert Patton and reports the information. The director asks the doctor a number of questions regarding Lockett's condition:
The warden "responded the doctor was checking the offender's heart beat and found a faint heart beat and the offender was unconscious," according to the timeline.
-- At 7:06 p.m., Lockett is declared dead.
There is nothing in the timeline that echoes the statements of witnesses, primarily journalists, who said 16 minutes into the execution Lockett was writhing in pain and appeared to be conscious.
According to Courtney Francisco of CNN affiliate KFOR, Lockett seemed to try to get up and to talk, saying "man" aloud.
Other reporters -- including Cary Aspinwall of the Tulsa World newspaper -- claimed that Lockett was "still alive," having lifted his head while prison officials lowered the blinds, blocking the view of the proceedings.
Charles Warner was scheduled to be executed later that night at the same prison. His execution has been postponed, at least temporarily.
Lockett and Warner 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. The department later disclosed the substances.
Lockett and Warner also took 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, ruling the two men had no right to know the source of the drugs intended to kill them.
CNN's Dana Ford, Greg Botelho and Ross Levitt contributed to this report.