Georgia prosecutors who spent more than two decades trying to execute Troy Davis have won what may be their final legal battle in the U.S. Supreme Court, yet state prison officials still can't schedule his execution.

This time it's because federal regulators seized the state's entire supply of a key lethal injection drug.

The Supreme Court's Monday decision to reject Davis' appeal clears the way for state authorities to execute Davis, who had been given a rare chance to argue his innocence but failed to convince a federal judge he was wrongly convicted of the 1989 murder of a Savannah police officer.

The high court's decision came at an inopportune time for Georgia authorities, who have set three previous execution dates for Davis since 2007 only to have each postponed so judges could review the case. Federal regulators this month seized the state's entire stockpile of sodium thiopental, a sedative used in the three-drug lethal injection cocktail, amid questions about how the state obtained it.

That means prison officials can't schedule Davis' execution until the Drug Enforcement Administration concludes its investigation or Georgia switches to another drug. Arizona, Texas and Ohio have already switched to another sedative, pentobarbital, amid a nationwide supply shortage, but Georgia prison officials won't comment on whether they are considering a similar move.

Still, the court's rejection is a crushing setback for Davis, who has become a cause celebre for the international anti-death penalty movement amid claims that he wasn't the one who killed off-duty Savannah police officer Mark MacPhail _ and that he had evidence to prove it.

Even Davis' attorneys acknowledge his options are limited. Defense attorney Jason Ewart said the likeliest _ and perhaps only _ route is the Georgia Board of Pardons and Paroles. The five-member panel has the power to commute or postpone executions, but rarely does so.

"We have to address the parole board. They said they wouldn't execute someone if there's doubt, and this case is so riddled with doubt," said Martina Correia, Davis' sister. "It's a shame in the U.S. when people don't value innocence. You would think the highest court in the land would have a lot more sensitivity to these issues."

MacPhail's mother, Anneliese MacPhail, said she hopes the court's decision has put to an end to Davis' legal appeals.

"I'm relieved that it's over now," said Anneliese MacPhail. "Well, maybe. I'm not believing it until it's over. It's been going on for so many years now that every time we think we're near the end, something else comes up. I just want this to end so badly, you won't believe it. This has been a nightmare."

MacPhail was working off-duty at a Savannah bus station on Aug. 19, 1989, when he was shot twice after rushing to help a homeless man who had been attacked. Eyewitnesses identified Davis as the shooter at his trial, but no physical evidence tied him to the slaying. Davis was convicted of the murder in 1991 and sentenced to death.

Davis has long claimed he could clear his name in MacPhail's death if a court would give him the chance to hear new evidence. The U.S. Supreme Court in 2009 agreed he should be able to argue his innocence, a rare chance afforded no other American death row defendant in at least 50 years.

During two days of testimony in June, U.S. District Judge William T. Moore Jr. heard from two witnesses who said they falsely incriminated Davis and two others who said another man had confessed to being MacPhail's killer in the years since Davis' trial.

But Moore concluded in August that several of the witnesses had already backed off their incriminating statements during the 1991 trial _ so it wasn't new evidence _ and that others simply couldn't be believed. He ruled that while the evidence casts some additional doubt on the conviction, "it is largely smoke and mirrors" and not nearly strong enough to prove Davis' innocence.

Davis sought to appeal Moore's decision, but the 11th Circuit Court of Appeals refused to hear the challenge in November. The Supreme Court then rejected the appeal on Monday, offering no elaboration.

Prosecutors were relieved by the decision. Lauren Kane of the Georgia Attorney General's office said it's consistent with every other court that reviewed Davis' case.

"Perhaps, now justice can finally be served in this matter," she said.

Supporters of Davis quickly called on the pardons board to commute his sentence.

Laura Moye of Amnesty International said lingering concern about the case "leaves an ominous cloud hanging over an irreversible sentence." And Kathryn Hamoudah, who chairs Georgians For Alternatives to the Death Penalty, said Davis' case exemplifies the problems with the state's death penalty system.

"Proceeding with the execution of Troy Davis would be callous, careless and irreversible," she said. "The state should be slowing down to address the well-documented, serious problems with a system that irreversibly takes human life, rather than rush to carry out an execution of a possibly innocent man."

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Associated Press Writer Mark Sherman in Washington contributed to this report.