The US Supreme Court is under increasing pressure to consider halting death sentences by lethal injection after a convicted Arizona killer took two hours to die.
Opponents and supporters of capital punishment alike say they believe the highest US court will agree to consider the issue.
Double murderer Joseph Wood gasped and snorted as he lay on a gurney for 117 minutes after officials from the US state of Arizona injected him with a relatively untested lethal drug cocktail.
His was the third execution this year to have been drawn out far beyond the usual 10 minutes, as states face a shortage of tried and tested drugs after European manufacturers halted exports.
Arizona Department of Corrections Director Charles Ryan on Thursday refuted suggestions Wood's execution had been "botched", insisting he was "comatose and never in pain" during the procedure.
But amid growing international uproar, opponents of the death penalty allege that the lengthy execution of Wood and others amount to a form of torture or the "cruel and unusual" punishment forbidden by the US Constitution.
"It is one of a pattern of botched executions that has resulted from an unacceptable level of secrecy on the part of the Department of Corrections," said Law Professor Deborah Denno of Fordham University.
"You're likely to see the Supreme Court more involved in examining this issue," predicted Steven Hall, director of The StandDown Texas Project, a legal rights pressure group opposed to capital punishment.
- Drug cocktail -
The court has rejected several appeals by inmates concerned by the procedure, including Wood, who was convicted of shooting dead his 29-year-old former girlfriend Debbie Dietz and her father Gene, 55.
Lawyers defending the 55-year-old Wood had sought more information about the state's lethal injection method, the executioner's qualifications and the manufacturer of the drugs.
The drug cocktail used in his execution -- a combination of the sedative midazolam and the painkiller hydromorphone -- had only been used once before -- in January in Ohio, where it took inmate Dennis McGuire 26 minutes to die.
In April, convicted killer and rapist Clayton Lockett was executed by lethal injection in the state of Oklahoma, in a process that took 43 minutes and during which he appeared to suffer before dying.
- 'Change is needed' -
"This has to stop -- we cannot be torturing, putting people on a gurney for two hours hoping that they die," said Richard Dieter, who heads the Death Penalty Information Center.
"This is a clear indication that change is needed at the highest level," he said.
Some 18 US states have abolished the death penalty, but others -- and the federal government -- maintain the practice, and polls suggest it retains majority public support.
But the debate has become more urgent since the states were faced with a critical shortage of approved execution drugs.
European drugmakers now refuse to supply products used to execute inmates, and US firms either lack the patents to produce them or do not want their brand associated with the controversy.
States have relied increasingly on compounding pharmacies, which lack federal approval and have had mixed success, to combine drugs to produce lethal doses.
The pharmacies that mix the drugs, fearing lawsuits or bad publicity, have insisted on secrecy, while lawyers for death row inmates battle in the courts to expose their methods to outside scrutiny.
News of the deaths has also tarnished the United States' international reputation.
A European Union spokesperson called for a moratorium and urged Arizona authorities "to follow the positive example of so far 18 US States who have abolished the death penalty."
But for death penalty supporters like Robert Blecker, a professor at the New York Law School, the controversy has been created by groups that would oppose executions however they are carried out.
"The lethal injection controversy is largely the work of abolitionist opponents," the author of "The Death of Punishment" said.
"They deny the traditional drugs by threatening the companies that are supplying them, then they challenge states."
Blecker, who would like to see the lethal injection replaced by a method that is clearly a punishment and not a medical procedure, alleged that campaigners would try to use the controversy to open a debate on the death penalty itself.