It ended as a nightmare for everyone involved, with the convicted murderer appearing to regain consciousness and struggling to sit up, prison officials halting the execution, and Lockett then dying of a massive heart attack.
Here's a look at what happened at the Oklahoma State Penitentiary on Tuesday night and some of the history behind it:
What went wrong?
It's not entirely clear. The prison director said an intravenous line blew after Lockett was given midazolam, a sedative, and while two other drugs — the paralytic vecuronium bromide and the heart-stopper potassium chloride — were flowing into his body.
Vein problems are not unheard of during executions, but Lockett's appeals team says his veins were perfectly healthy. They suspect there was something wrong with the drugs or the way they were administered.
Lockett's lawyers say the injection was "experimental." Is that true?
The three-drug combination had been used in Florida, but never in Oklahoma. And Oklahoma chose a different dosage of midazolam, a common sedative known by the brand name Versed, so it's accurate to say the exact formula used Tuesday night had not been tested.
Before the execution, Lockett's lawyers tried to force prison officials to reveal where they obtained the lethal dose, saying they should be allowed to investigate how they were prepared and whether they would be effective, but the state's high court ultimately ruled against them.
Why is Oklahoma using a new cocktail?
The state retooled its protocol after announcing it had not been able to find the drugs for its previous lethal injection combo: pentobarbital and vecuronium bromide.
That's a scenario that has repeated itself across the country as the supply of execution chemicals has dwindled.
Some pharmaceutical companies have refused to sell their products for capital punishment, forcing prisons to turn to less-regulated compounding pharmacies for specially mixed injections. In the wake of bad publicity and legal hassles, some pharmacies have gotten out of the business, too.
States are trying to keep the supply lines open by giving their drug connections anonymity, but defense lawyers contend the secrecy prevents them from investigating whether the injections would violate the constitutional protection against cruel and unusual punishment.
Is Lockett's execution the first one to go wrong?
At least two recent executions using new cocktails have drawn scrutiny because of complications.
When Michael Lee Wilson, 38, was put to death in January in Oklahoma for the murder of a store clerk, he reportedly blurted out, "My whole body is burning," after the pentobarbital was injected.
In Ohio, Dennis McGuire, 53, took 25 minutes to die and appeared to gasp for breath in January when given an untested cocktail that included midazolam. The state put another execution on hold while it reviewed what was seen by some as a botched procedure.
Earlier this week, Ohio declared that the execution was "humane" and that McGuire — convicted of raping and murdering a pregnant woman — felt no pain.
What happens now?
Oklahoma Gov. Mary Fallin — who last week accused the state Supreme Court of overstepping its bounds by putting Lockett's execution on temporary hold — said she has ordered a review of his death.
His lawyers are calling for an independent autopsy and the names of the drug sources, though it remains to be seen if that will happen.
The man who was scheduled to die two hours after Lockett, Charles Warner, received at least a 14-day reprieve.
Nationally, death-row inmates continue to challenge changes to execution protocols and laws and policies that keep the suppliers a secret.
Lawmakers in some death-penalty states are also pushing to have other forms of execution — such as firing squad or electric chair — reinstated. Experts on both sides of the debate say it's unlikely that will come to pass on a broad scale because the public is unlikely to embrace non-medical executions.
Some death-penalty foes are predicting the high-profile Lockett disaster — in the wake of publicity about reversed convictions — could erode American support for the death penalty.
What does the U.S. Supreme Court have to say about it?
While several justices indicated they were in favor of a stay of execution in a couple of cases where inmates challenged the drug secrecy, the nation's high court has yet to block an execution because of that issue.
Three weeks ago, the justices declined to take up the case of Christopher Sepulvado, who was sentenced to death for killing his 6-year-old stepson and challenged Louisiana's secrecy rules.