A death-row inmate executed for the murder of a woman and the rape and murder of a teenage girl suffered "excruciating" pain "similar to drowning" during the lethal injection procedure.
Wesley Ira Purkey was put to death last month by an injection of pentobarbital in Terre Haute federal penitentiary, Indiana.
An autopsy performed on Purkey revealed that he suffered "severe bilateral acute pulmonary oedema" and "frothy pulmonary oedema in trachea and main stem bronchi".
This means fluid quickly entered Purkey's lungs and trachea, causing "a near-drowning" sensation which a medical expert described as "among the most excruciating feelings known to man".
Purkey was sentenced to death in 2004 for the teenage girl murder, after previously being sentenced to life for a murder which earned him the moniker, the "claw hammer killer".
Purkey was convicted and sentenced to death for kidnapping, raping and killing 16-year-old Jennifer Long in 1998.
Long was last seen at East High School in Kansas City, Missouri on January 22 that year, when she skipped lessons.
Purkey lured the teenager into his pick-up truck outside a grocery store.
At the time Purkey, aged 46, was high on crack and drove the girl to his home where he raped her and stabbed her to death.
Using an electric chainsaw, he dismembered Long's body, and partially burned it in his fireplace.
He then took her remains and drove them 300km to a septic pond in Clearwater, Kansas, where he dumped them.
Jennifer Long's remains have never been found.
Nine months later, in October 1998, Purkey beat an 80-year-old polio sufferer to death with a claw hammer.
Purkey had been working for a plumbing company and had gone to the home of Mary Ruth Bales to fix a kitchen tap.
Neighbours saw him trying to burn the woman's body and he was arrested.
Purkey pleaded guilty to Bale's murder and was sentenced to life in prison where, years later, he confessed to Long's murder and was given the death penalty.
The execution came after the US Supreme Court rejected Purkey's plea that he had dementia.
Long's father and stepmother attended the execution, with William Long saying, "I hope he rots in hell."
Purkey's execution was carried out two days after the US government conducted its first federal execution in 17 years, also at Terre Haute, of white supremacist Daniel Lewis Lee.
Lee was convicted of killing an Arkansas family in a 1990s plot to build a whites-only America.
The autopsy, which is unofficial, was carried out at the behest of Purkey's relatives.
The results were obtained by a third Terre Haute inmate who is scheduled to be the next federal prisoner to be put to death.
Keith Nelson, convicted in the 1999 rape and strangulation of 10-year-old Pamela Butler, filed the autopsy claims in a court bid to have his execution stayed.
Nelson abducted Butler, pulling her into his truck, as she rollerbladed back to her Kansas home after buying cookies.
Dr Gail Van Norman, a medical expert retained by Nelson's lawyers to interpret Purkey's autopsy, said the flash flood-like filling of Purkey's lungs could only occur when a person was still alive.
"It is a virtual medical certainty, that most, if not all, prisoners will experience excruciating suffering, including sensations of drowning and suffocation from (the drug) pentobarbital," she said.
Nelson's execution is set for August 28.
The execution of Lezmond Mitchell, the only Native American on federal death row, is scheduled for this Wednesday, August 26.