(CNN) -- Complications with the placement of an IV into a vein of death row inmate Clayton Lockett played a significant role in his botched execution, according to a report by the Oklahoma Department of Public Safety.
Lockett was executed by the state of Oklahoma on April 29 in a lethal injection protocol that was beset with complications.
An autopsy confirmed that Lockett died from the execution drugs and not from a heart attack, but many consider it botched nonetheless because it took 43 minutes for him to die.
Oklahoma Gov. Mary Fallin ordered an investigation into the execution, which thrust the debate over capital punishment back into the spotlight.
The report was released Thursday.
Oklahoma Department of Corrections Director Robert Patton told Fallin that his agency will rewrite execution protocols, Fallin said.
"I continue to believe the death penalty is an appropriate and just punishment for those guilty of the most heinous crimes, as Mr. Lockett certainly was. The state's responsibility is to ensure a sentence of death is carried out in an effective manner," Fallin said.
One factor that led to the complications was the difficulty the doctor and paramedics in the execution chamber had in placing the IV that would deliver the execution drugs into Lockett's veins.
It took 51 minutes and attempts in different parts of his body before the doctor felt comfortable with the IV placed in Lockett's right groin area, the report said. For placement there, the doctor told investigators he would have preferred a longer needle, but the size he wanted was not available, so he used what was available.
Because Lockett's groin area was exposed to place the IV, Warden Anita Trammell decided to cover up the exposed area with a blanket to protect the inmate's dignity, the report said.
As a consequence, the place where the IV was inserted was covered and not visible to the doctor.
It wasn't until the second and third drugs of the execution cocktail were being administered that the doctor saw something was wrong, the report said.
"Lockett began to move and make sounds on the execution table," the report states. "It should be noted that the interview statements of the witnesses regarding Lockett's movements and sounds were inconsistent."
Lifting the blanket, those in the execution chamber observed clear liquid and blood on Lockett's skin near the groin area, and swelling under the skin that was "smaller than a tennis ball but larger than a golf ball," the report states.
If the doctor would have been able to see the point where the IV was inserted, the problem could have been identified sooner, according to the report.
At that point, according to the report, the doctor was unsure how much of each drug made it into the vein, or whether it was enough to result in death.
The doctor was unable to insert the IV in the other femoral artery. Another failure noted in the report was that even if another vein was used, there was not a backup supply of execution drugs if indeed an insufficient amount had been injected the first time.
The report notes that as the team struggled with these complications, Lockett's heartbeat continued to lower.
The execution was halted, but Lockett died anyway.
The investigation found that there were some minor deviations from the required protocol during the execution, but that those did not contribute to the complications.
"This investigation concluded the viability of the IV access point was the single greatest factor that contributed to the difficulty in administering the execution drugs," the report states.
Among the report's recommendations was to make the IV insertion point visible to the doctor at all times and to alert the governor's office if it takes more than one hour to insert the IV.
The incident provoked sharp criticism from attorneys and lethal injection experts from the Death Penalty Clinic at the University of California Berkeley School of Law.
"The state's internal investigation raises more questions than it answers," said Dale Baich, an attorney representing Oklahoma death row prisoners. "The report does not address accountability. It protects the chain of command. Once the execution was clearly going wrong, it should have been stopped, but it wasn't."
Megan McCracken of the Death Penalty Clinic called for an independent investigation.
"Multiple factors contributed to Mr. Lockett's badly bungled execution, including lacking of planning and training, a failure to set a functioning IV, and a failure to plan for contingencies. Many questions remain unanswered as to how the (Department of Corrections) allowed this to happen," McCracken said in a statement.
CNN's Carma Hassan and Michael Martinez contributed to this report.