Jennifer Joyce, a St Louis district attorney, has decided to reopen the case of Larry Griffin, who was convicted in 1981 of the murder of Quintin Moss, a 19-year-old drug dealer who was shot dead. Griffin maintained his innocence to the end, but was put to death in 1995, aged 40, by lethal injection.
Since then the first police officer at the scene, another witness to the shooting and the victim's family have all raised doubts that Griffin was guilty.
The principlal witness for the prosecution, a convicted drug dealer, changed his account shortly before the execution but has since died.
There have been dozens of cases of prisoners on death row having their convictions overturned, but the innocence of someone who has been executed has not been proved since the death penalty was reintroduced in 1976.
If Ms Joyce were to overturn the conviction posthumously it would provide a huge boost to anti-death penalty campaigners.
"What I have heard recently is very troubling and leads me to believe an innocent man was executed for this murder, while the real killers have not been brought to justice," said Missouri congressman William Clay.
Pressure to reopen the case came from America's oldest civil rights organisation, the NAACP, following a report by the University of Michigan Law School.
"If they prove he was innocent, that would be the gold standard," Joshua Marquis, an advocate of the death penalty, told the New York Times.
"I'm not sure opponents of the death penalty would start prevailing, but they'd be able to say to people like me, 'What about Mr Griffin?'"
The murder took place on June 26 1980 when Moss was shot dead from a slow-moving car.
Central to the push to establish Griffin's innocence are accusations of flawed credibility and testimony of the only eyewitness involved in the trial.
Robert Fitzgerald, a career criminal from Boston who said he got a good look at the gunman, remembered his licence plate numbers and later identified Griffin in photographs as well as the abandoned car.
Shortly after Griffin was convicted, Fitzgerald was released and cleared on felony and fraud charges.
Fitzgerald later changed his story saying that he had only been shown one picture and told: "We happen to know who did it."
Moreover, nobody else who was on or near the scene remembers Fitzgerald being there - a strange oversight since he was white and this was a black area.
The first police officer at the scene says he never saw a white man, although testified otherwise at the trial.
A man who was struck by a stray bullet but never called upon to testify at the trial says Fitzgerald was not there.
"Fitzgerald was the entire case and now there's very strong eyewitness evidence that Fitzgerald was not there and what's more, Larry Griffin was not there," said Samuel Gross, a law professor at the University of Michigan who conducted the NAACP's investigation.
However, the original prosecutor in the case, Gordon Ankney, cites other facts as proof that he got the right man.
Mr Ankney claims an off-duty officer said Griffin got in the car that was used in the shooting on the day of the murder and that the murder weapon was found in the car.
"I believe the jury did the right thing, and nothing's happened that's led me to believe otherwise," Mr Ankney told Associated Press.