The 14 December 2006 execution of inmate
Angel Diaz in Florida took a total of 34 minutes, and a second round of injections to complete.
Witnesses to the execution reported that Diaz "gasped" and "grimaced" during the procedure.
An autopsy revealed chemical burns on Diaz's arms, where needles had been pushed all the way through veins and into soft tissue.
Jeb Bush, Florida's governor at the time, temporarily suspended state executions the following day.
A paper later stated that the inmate probably died from "progressive suffocation" caused by the nerve-blocking drug, and that he was likely to have experienced "potassium-induced sensation of burning".
The team of lawyers and medical experts who wrote the paper concluded: "The conventional view of lethal injection leading to an invariably peaceful and painless death is questionable."
Dr Jay Chapman, who first approved the triple-drug cocktail in 1977 when he was Oklahoma's state medical examiner, has now endorsed the single drug method, which uses a single high dose of anaesthesia - the same method used to put pets to sleep.
Dr Chapman told BBC News: "A one-drug protocol is employed by veterinarians for euthanasia simply because the procedure is humane."
The one-drug method, which takes longer to end life than the three-drug system, was not considered when lethal injection was first devised. The slower time was seen as less humane.
Austin Sarat, a professor of jurisprudence at Amherst College in Massachusetts says that lethal injection, like electrocution, hanging and lethal gas before it, has been shown to have "flawed technology" that prevents the guarantee of a painless death every time.
"In hanging, this means people were strangled rather than having their necks broken," he says. "There have been cases where people [in the electric chair] burst into flames.
"Three drugs didn't do it, now we're at one. We need to kill people softly, to kill people gently in order to have a legitimate form of execution. And we just can't figure out what that is."
Source: BBC News, August 8, 2012