(CNN) -- The human rights group Amnesty International renewed its call for clemency Wednesday for a man on Georgia's death row, citing continued doubts about his guilt.
Troy Davis, 42, is set to be executed on September 21.
Davis "could very well be innocent," said Laura Moye, the head of Amnesty's Death Penalty Abolition Campaign. "It's difficult to believe that a system of justice could be so terribly flawed."
In March, the U.S. Supreme Court rejected Davis' request that his execution be delayed in order to gain more time to prove his "actual innocence."
Davis has gained international support for his long-standing claim he did not murder an off-duty Savannah police officer more than two decades ago. He was granted a stay of execution by the Supreme Court two hours before he was to be put to death in 2008, and the court in 2009 ordered the federal district court to take another look at the case.
That court, after holding a hearing to review evidence, ruled in August 2010 that Davis "failed to show actual innocence" in the case.
Witnesses have said Davis, then 19, and two others were harassing a homeless man in a Burger King parking lot in 1989 when off-duty officer Mark MacPhail came to the man's assistance. They testified that Davis shot MacPhail twice and fled.
Since Davis' conviction in 1991, seven of the nine witnesses against him have recanted their testimony. No physical evidence was presented linking Davis to the killing of the policeman.
But upon reviewing Davis' claims of innocence, the U.S. District Court for the Southern District of Georgia found last year that Davis "vastly overstates the value of his evidence of innocence."
"Some of the evidence is not credible and would be disregarded by a reasonable juror," Judge William T. Moore wrote in a 172-page opinion. "Other evidence that Mr. Davis brought forward is too general to provide anything more than smoke and mirrors," the court found.
Prominent figures ranging from the pope to the musical group Indigo Girls have asked Georgia to grant Davis a new trial.
Other supporters include celebrities Susan Sarandon and Harry Belafonte, world leaders such as former President Jimmy Carter and former Archbishop Desmond Tutu, and former and current U.S. lawmakers Bob Barr, Carol Moseley Braun and John Lewis.
CNN's Bill Mears and Tom Watkins contributed to this report