tates "have intensified their efforts to obscure information regarding the development and implementation of their lethal injection protocols," said the report by a committee of experts convened by the Constitution Project, a non-profit group that promotes bipartisan consensus on legal reform.
"This poses an unacceptable risk that inmates will face an unnecessarily cruel and painful death, violative of the US Constitution," the 208-page study said, calling for comprehensive US capital punishment reform.
Clayton Lockett, a convicted murderer and rapist, was administered a new, untested three-drug protocol on April 29 in Oklahoma. He died 43 minutes after the start of the injection -- the process usually takes 10 minutes -- prompting allegations of torture.
Lawyers had pleaded in vain in court demanding information about the new procedure, in particular the composition of the drugs and where they came from. But Oklahoma refused to provide such information.
"Such secrecy undermines the public's faith in the integrity of the justice system," the study said, calling for lethal injection protocols to be "handled in a transparent manner."
A death row inmate in Texas whose execution is scheduled for May 13, meanwhile, has appealed for a stay of execution and has also demanded to be informed of such information.
The study calls for "public review and comment" of any new lethal injection procedure and transparency concerning "the nature, characteristics and origins of the specific drugs used."
After Lockett's execution, President Barack Obama called the ordeal "deeply troubling" and warned that it raised "significant questions about how the death penalty is being applied."
The state of Oklahoma called for a review of the procedure, with multiple entities demanding impartiality.
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The report, which made 39 recommendations, demanded that procedures be put in place to minimize the risk of people suffering.
The committee of experts said they believe that the Supreme Court decision Baze v. Rees, which deemed lethal injections constitutional in 2007, is up for question today because the products being used are no longer the same.
Since European manufacturers began refusing to sell the most commonly used anesthetic -- pentobarbital -- for human executions, the 32 US states that impose capital punishment "modified virtually any aspect of their lethal injection procedures with a frequency that is unprecedented among execution methods in this country's history," said Deborah Denno, professor of law at Fordham University.
"There have been more changes in lethal injection protocols during the past five years than there have been in the last three decades," she said.
"States have turned to increasingly non-traditional sources of drugs, such as compounding pharmacies, resulting in overwhelming criticism and legal challenges," she said.
Meanwhile, the companies are under the authority of the very states that are demanding the drugs, which are not approved at the federal level.
States "should use only drugs obtained in compliance with all laws and approved by the US Food and Drug administration" the report said.
Products prepared at a Massachusetts compounding pharmacy gained particular notoriety when they were implicated in a deadly meningitis outbreak in November 2012.
Prison authorities should also assure the presence of qualified medical personnel during executions, the report said, because such procedures are "susceptible to error."
The report was written before Lockett's death and therefore does not mention his botched execution.
AFP