Prison authorities are increasingly drawing a curtain over their scramble for execution drugs. AP Photo/Ohio Dept. of Rehabilitation and Corrections

A condemned Ohio inmate appeared to gasp several times and took more than 15 minutes to die Thursday as he was executed with a combination of drugs never before tried in the U.S.

Dennis McGuire's attorney, federal public defender Allen Bohnert, called his client's death "a failed, agonizing experiment by the state of Ohio."

McGuire's son, also named Dennis, and daughter, Amber, said Friday that their father's unusually slow execution amounted to torture, and announced plans to sue over his death.

McGuire made loud snorting noises during one of the longest executions since Ohio resumed capital punishment in 1999.

His attorneys attempted to halt the execution last week, arguing that the untried method put him at substantial risk of "agony and terror" while he strainedto catch his breath in a medical phenomenon known as air hunger.

Amber McGuire said she was so horrified that she covered her ears so she wouldn't hear the sounds her father made, describing it as “torture.”

“I don’t feel like anybody deserves that — families, or my dad, anybody on death row — nobody deserves to go through that,” said her brother.

Ohio prisons spokeswoman JoEllen Smith had no comment on how the execution went but said a review will be conducted as usual.

David Waisel, a professor of anesthesia at Harvard University, told Al Jazeera that on the basis of the execution reports, he thinks McGuire’s snorting sounds indicate that the inmate wasn’t completely asleep while the drugs were taking effect.

Waisel described the feeling of air hunger as “the incredible need to take a breath and being unable to. You’re starving to breathe.” He compared it to playing sports and having the air knocked out of you. “It’s an awful feeling.”

The state used intravenous doses of two drugs, the sedative midazolam and the painkiller hydromorphone, to put McGuire to death for the 1989 rape and fatal stabbing of a pregnant woman, Joy Stewart. The method was adopted after supplies of a previously used execution drug ran out because the manufacturer put it off limits for capital punishment.

The new execution drug cocktail — which uses two chemicals in short supply among hospitals in the U.S., according to the American Society of Health-System Pharmacists — was chosen as prison authorities scramble to find replacement lethal-injection ingredients. Pharmaceutical firms in Europe and the United States have increasingly objected to their products’ use in the death chamber, leading to new mixes, often purchased from semiregulated compounding pharmacies.

Maya Foe of the legal-rights charity Reprieve described the execution as “shocking,” adding that it was tantamount to “human experimentation at its most cruel and unusual.”

“Ohio was warned by leading experts that experimenting on people in this way risked causing them serious suffering,” she said, “and the evidence suggests that this has been borne out. How many more botched executions do we need to see before executioners stop using humans as guinea pigs?”