Prison officials in Arizona have denied inmate Joseph Rudolph Wood was in pain during execution yesterday afternoon.
Wood gasped for around 90 minutes during his execution and eventually died almost two hours after the lethal drugs where administrated. His lawyers filed an emergency appeal while the execution was ongoing.
State Departments of Corrections Director Charles Ryan said he was assured "unequivocally" that Mr Wood was aware of what was happening, or in any pain.
A full review has been ordered into the execution of double murderer Joseph Wood, who took two hours to die after being given a lethal injection.
Arizona's Governor Jan Brewer said that while justice had been done she was concerned by how long the lethal injection procedure took.
She said: "One thing is certain, however, inmate Wood died in a lawful manner and by eyewitness and medical accounts he did not suffer.
"This is in stark comparison to the gruesome, vicious suffering that he inflicted on his two victims, and the lifetime of suffering he has caused their family."
A death row inmate "gasped and snorted" as he took two hours to die by lethal injection in the latest "botched" execution in America, Reuters reported.
Double killer Wood, 55, was given a lethal injection of drugs at 1.52pm local time but was not pronounced dead until 3.49pm, the Arizona attorney general's office said.
Prior to his execution Wood was one of six death row inmates who sued Arizona arguing secrecy surrounding the drugs used in other botched executions in Ohio and Oklahoma violated their human rights.
His death raises questions about the use of the death penalty in the US.
The White House has said the botched execution of a death row prisoner in Oklahoma fell short of the humane standards that must apply to carrying out the death penalty.
The execution of Clayton Lockett was stopped when he regained consciousness after receiving a lethal injection, before dying from a heart attack around 30 minutes later.
ITV News Washington Correspondent Robert Moore reports.
The drugs used in a botched lethal injection in Oklahoma were the subject of an appeal from lawyers representing two death row inmates.
The execution of Clayton Lockett was stopped after the drugs he was administered "weren't working as designed", prison officials said.
Lawyers for Lockett and fellow inmate Charles Warner had argued before the botched execution that the use of an untested cocktail of drugs could subject their clients to "cruel and unusual punishment", which is prohibited by the US constitution.
Oklahoma's governor has announced a new evaluation of the state's procedures for lethal injections following the botched execution of an inmate earlier.
The drugs used in the botched lethal injection of Clayton Lockett "weren't working as they were designed to", a prison spokesman has said.
Jerry Massie told reporters:
"We believe that a vein was blown and the drugs weren't working as they were designed to. The director ordered a halt to the execution."
Oklahoma inmate Clayton Lockett apparently called out during his execution, saying "something's wrong", an Associated Press reporter on the scene has said.
Lockett later died of a heart attack, but the execution of fellow inmate Charles Warner has been postponed for 14 days.
Officials in the US state of Oklahoma have reportedly postponed the second of two executions set for today after the first lethal injection was botched.
The Associated Press said the execution of Clayton Lockett was stopped after he apparently regained consciousness.
Lockett then suffered a heart attack and died around half an hour after officials halted the execution.
Now the execution of Charles Warner, which had been due two hours after Lockett's, will be stayed for a fortnight.