A US murderer struggled and gasped for air for at least 10 minutes as he was put to death in a prolonged execution using a controversial new drug cocktail, according to witnesses.
A journalist for the Columbus Dispatch newspaper present at the execution of 53-year-old Dennis McGuire reported that the Ohio killer made "snorting and choking" sounds as he succumbed.
Ohio officials told AFP that McGuire, sentenced to death in 1989 for the rape and murder of a young pregnant woman, was declared dead at 10:53 am (1553 GMT).
Journalists who witnessed the execution said the drugs used in the lethal injection had begun to be administered 24 minutes earlier at the prison in Lucasville.
Under a new Ohio protocol, McGuire was executed using a cocktail comprising the sedative midazolam and analgesic hydromorphone, a combination never previously used in the United States.
The new execution protocol was introduced after Ohio and other US states that retain the death penalty began running out of barbiturates when European manufacturers stopped supplying them.
McGuire's lawyers had opposed the method of execution, saying the killer would die of asphyxia in a phenomenon known as "air hunger," inflicting the sort of cruel and unusual punishment prohibited under the US Constitution.
But appeals, which went all the way to the US Supreme Court, were rejected.
A federal judge in Ohio, Gregory Frost, said "the evidence before the court failed to present a substantial risk that McGuire will experience severe pain."
Journalists who witnessed Thursday's execution said McGuire appeared to be suffocating as he was put to death.
According to the pool of reporters, it was the longest execution since Ohio re-introduced the death penalty in 1999.
"At about 10:33 am, McGuire started struggling and gasping loudly for air, making snorting and choking sounds that lasted for at least 10 minutes, with his chest heaving and his fist clenched," the Columbus Dispatch reported.
"Deep, rattling sounds emanated from his mouth. For the last several moments before he was pronounced dead, he was still."
Death penalty experts voiced concern at the details of McGuire's death, saying it pointed to a method of execution which was "egregious and problematic."
"In light of the length and disturbing descriptions of Dennis McGuire’s execution, in addition to the range of lethal injection complications reported in other states, it appears that this country's lethal injection procedure is more egregious and problematic than it ever has been," Deborah Denno, from the Fordham University School of Law, told AFP in an email.
Maya Foa, director of the death penalty team at the Reprieve non-profit group, accused Ohio authorities of ignoring expert advice.
"Ohio was warned by leading experts that experimenting on people in this way risked causing them serious suffering, and the evidence suggests that this has been borne out," Foa said in a statement.
"How many more botched executions do we need to see before executioners stop using humans as guinea pigs?"
Thursday's execution was the second carried out this year by US authorities using new products which appeared to result in suffering of the condemned.
On January 9 in Oklahoma, condemned killer Michael Lee Wilson said he could feel his "whole body burning" as he was put to death.
Wilson was executed using a mix of drugs including pentobarbital, a substance commonly used to euthanize animals.
However, these kinds of drugs are now only produced by pharmacies that are governed by local laws, rather than federal regulators.
A scandal erupted in November 2012 in Massachusetts when poor hygiene at one of these pharmacies was blamed as the cause of a deadly meningitis outbreak.
McGuire is the third man to be executed in the United States this year. Ohio executed three of the 39 people put to death in the United States last year, according to the Death Penalty Information Center.