A double execution was interrupted after lawyers claimed the first of two men scheduled to be killed in the US state of Arkansas was still moving more than five minutes after he received a fatal injection, news.com.au reports.
The shock stay lasted about an hour before it was lifted to allow the state's first double execution in more than 16 years to go ahead.
The decision came as a remorseful note written by the executed rapist and murderer, who was reportedly "gulping for air" while he died, has emerged.
Jack Jones was given the injection and pronounced dead at 7.20pm Monday (local time).
Marcel Williams, who was also put on death row for rape and murder, was pronounced dead at 10:33 p.m local time.
Following Jones's execution, lawyers for Williams claimed officials spent 45 minutes trying to place an IV line in Jones's neck before placing it elsewhere. In a last-minute appeal, they said Jones was still conscious, moving his lips and "gulping for air" after being administered with the sedative midazolam that is supposed to render inmates unconscious, according to local media reports.
The state's attorney general's office disputed Williams's legal team's account, and US District Judge Kristine Baker decided the punishment would go ahead.
According to earlier reports, the last words of Jack Jones, the rapist and murderer pronounced dead at 7.20pm Monday local time, were: "I'm sorry."
"I hope over time you can learn who I really am and I am not a monster," he said in the roughly two-minute statement.
In a handwritten statement written just before his death, Jones said he was filled with remorse.
"I want people to know that when I came to prison I made up my mind that I would be a better person when I left than when I came in," he wrote.
"I had no doubt in my mind that I would make every effort to do this. I'd like to think that I've accomplished this."
Jones said he made "every effort" to be a good person, practising Buddhism and studying physics.
"There are no words that would fully express my remorse for the pain that I caused," he wrote.
Jones was one of eight death row inmates the state wanted to execute before the end of April while its stock of midazolam, one of the drugs used, was still in date.
But amid public opposition to the death penalty lawyers obtained stays on four of those executions.
One execution had already taken place last week and Jones, the first of two prisoners to be killed in today's double execution, was given the lethal injection after the US Supreme Court rejected his lawyers' plea for a stay.
Williams's execution has now been halted until the judge issues another order.
The two men were given last meals on Monday, Arkansas Department of Correction spokesman Solomon Graves said.
Jones had fried chicken, potato logs with tartar sauce, beef jerky bites, three candy bars, a chocolate milkshake and fruit punch.
Williams had fried chicken, banana pudding, nachos, two sodas and potato logs with ketchup.
Lawyers for both men had argued their poor health would make it difficult for them to respond during a consciousness check following the megadose of midazolam, and the state shouldn't risk giving them the drugs.
The US Supreme Court allowed the first of two executions set to go ahead, turning back Jones's claims that his poor health could cause excruciating pain during the lethal injection and a similar claim from Williams's lawyers is expected to be rejected.
Jones was given the death penalty for the 1995 rape and killing of Mary Phillips. He strangled her with the cord to a coffee pot.
In a letter earlier this month, he said he was ready to be killed by the state.
"I forgive my executioners; somebody has to do it," wrote Jones, who had a leg amputated in prison because of diabetes and uses a wheelchair.
The letter, which his lawyer read aloud at his clemency hearing, went on to say: "I shall not ask to be forgiven, for I haven't the right."
Williams was sent to death row for the 1994 rape and killing of 22-year-old Stacy Errickson, whom he kidnapped from a gas station in central Arkansas.
Authorities said Williams abducted and raped two other women in the days before he was arrested in Errickson's death. Williams admitted responsibility to the state Parole Board last month.
Before Ledell Lee's execution last Thursday, Arkansas hadn't put an inmate to death since 2005.
A fourth execution for the month is set for Thursday.
The initial eight executions that Jones and Williams's would have been part of would have been the most by a state in such a short period since the US Supreme Court reinstated the death penalty in 1976.
The state said the executions needed to be carried out before its supply of the sedative midazolam expires on April 30.
The last state to put more than one inmate to death on the same day was Texas, which executed two killers in August 2000. Oklahoma planned a double execution in 2014 but scrapped plans for the second one after the execution of Clayton Lockett went awry.
Arkansas executed four men in an eight-day period in 1960. The only quicker pace included quadruple executions in 1926 and 1930.
In several of the 31 states where executions are legal, drug shortages have often forced delays as manufacturers prohibit their use in executions.
Arkansas believes that secrecy it grants to suppliers can solve that problem, though it still has difficulty obtaining the drugs, and court-ordered rewrites of the state's lethal injection protocols have also caused delays.
"I wish I could take it back, but I can't," Williams told the board.