The botched Oklahoma execution of Clayton Lockett was largely due to a collapsed vein during the lethal injection, and the needle was inserted in the groin area instead of the arm after prison officials used a stun gun to restrain him, according to a prisons report said.
Department of Corrections Director Robert Patton said in the report the state's execution protocols needed to be revised and called for an indefinite stay of executions until the new procedures are in place and staff trained.
Ahead of last Tuesday's execution, Lockett, a convicted murderer, had refused to be restrained, the report said, and after being given a warning "an electronic shock device was administered," causing an injury to his arm.
The state has come under a barrage of criticism for the botched execution that many saw as a violation of constitutional protections against cruel and unusual punishment.
The White House said the process fell short of humane standards.
Lockett, 38, died of an apparent heart attack minutes after the lethal injection protocol failed.
He was convicted of shooting 19-year-old Stephanie Nieman in 1999 and then helping to bury her alive in a shallow grave, where she died.
Witnesses to the execution said Lockett clenched his jaw and fists a few minutes after the drugs were injected and appeared to be in pain.
Prison officials covered the windows to the death chamber soon after as it became apparent there was trouble.
The doctor overseeing the execution reported that Lockett's vein had collapsed during the injection and the drugs had either absorbed into the tissue, or leaked out or both, Mr Patton said.
Mr Patton asked if enough drugs been administered to cause death, and the doctor answered "no."
Mr Patton then asked if another vein was available and if enough drugs remained to finish the execution. The doctor responded "no" to both questions.
Once the problem became apparent, Mr Patton halted the execution.
Lockett died 43 minutes after the execution started, and an autopsy was under way.
"I intend to explore best practices from other states and ensure the Oklahoma protocol adopts proven standards," Mr Patton said in his report.
Mr Patton said in the report that medical officials examined Lockett's arms, legs, feet and neck for veins, but no viable entry point was located. After that, a lethal injection insertion point was used in Lockett's groin area.
"As the Oklahoma Department of Corrections dribbles out piecemeal information about Clayton Lockett's botched execution,they have revealed that Mr Lockett was killed using an invasive and painful method - an IV line in his groin," said Madeline Cohen, a lawyer who fought to halt the execution of another Oklahoma death row inmate.
That inmate, convicted murderer and rapist Charles Warner, had also been scheduled for execution on Tuesday night, but was granted a 14-day stay after the botched execution of Lockett.
The executions of Lockett and Warner, convicted in separate crimes, had been put on hold for several weeks because of a legal fight over the state's new lethal injection cocktail, with lawyers arguing Oklahoma was withholding crucial information about the drugs to be used.
Attorneys for death row inmates have argued that the drugs used in Oklahoma and other states could cause an unnecessarily painful death, in violation of the US Constitution.