On Tuesday, Georgia's Board of Pardons and Parole, which has the final say in such matters, rejected a clemency petition for Troy Davis, who is on death row for the 1989 killing of off-duty police officer Mark MacPhail. Davis, 42 — scheduled to be put to death by lethal injection at 7 p.m. on Wednesday — says he's innocent, and a long list of supporters — including Archbishop Desmond Tutu, Pope Benedict XVI, former FBI director William Sessions, Jimmy Carter, and Amnesty International — are sufficiently convinced to call for a halt to the execution. Is Georgia making a terrible mistake? Here's what you need to know:

What is Davis accused of?
Officer MacPhail, working as a security guard in his off-hours, was shot in the parking lot of a Burger King in Savannah, Ga., while attempting to help a homeless man who was being beaten with a gun. Prosecutors say Davis was the gunman, and had a smirk on his face as he shot MacPhail. He was convicted in 1991.

Why do people question his guilt?
First of all, seven of the nine witnesses who tied Davis to the crime have at least partly recanted their testimony — some claiming police duress influenced their initial testimony. One witness later said that a different man, Sylvester "Redd" Coles, had bragged afterward about shooting MacPhail. Not coincidently, says Dave Zirin at The Nation, "of the two witnesses who still maintain that Troy was the triggerman, one is Sylvester 'Redd' Coles." Also, police produced no murder weapon, blood, DNA, surveillance tape, or other physical evidence tying Davis to the crime. The only hard evidence were shell casings that prosecutors say matched a gun linked to Davis from another crime.

And Davis can't get a new trial?
No. In a statement, the parole board said it had "considered the totality of the information presented in this case and thoroughly deliberated on it, after which the decision was to deny clemency." Prosecutors have never doubted the conviction, and have prevailed in a string of "extraordinary" legal challenges, says CNN legal analyst Jeffrey Toobin. The pardons boards did stay his execution in 2007, then declined to extend the stay a year later. Two hours before Davis was to be executed in 2008, the U.S. Supreme Court stepped in — "something it almost never does," Toobin says — ordering a U.S. District Court to re-examine the case. Federal Judge William T. Moore ruled in 2010 that the new evidence "casts some additional, minimal doubt" on Davis' conviction, but not enough to order a new trial.

So is Davis guilty or not?
That's still a matter of much dispute. "He has had ample time to prove his innocence," says MacPhail's widow, Joan MacPhail-Harris. "And he is not innocent." Come on, says The Nation's Zirin. Davis is "a demonstrably innocent man that the state is about to execute in the premeditated manner of a murder." Look, says former Rep. Bob Barr (R-Ga.) at CNN, it's hard to know for sure if he's guilty or innocent. But there's plenty of doubt in the case, and "imposing a death sentence on the skimpiest of evidence does not serve the interest of justice."

Sources: APAtlanta Journal-ConstitutionCNN (2), GuardianThe NationReutersTelegraphUSA Today