ATLANTA — Attorneys for convicted cop killer Troy Davis are exploring legal options after his execution date was set for later this month, though they acknowledge his options are slim.

Davis was set to be executed Sept. 21 for the 1989 murder of Savannah officer Mark MacPhail, state officials said Wednesday. It marks the fourth time since 2007 an execution date was set for Davis, whose case has attracted worldwide attention from death penalty opponents such as former President Jimmy Carter and Pope Benedict XVI.

Davis exhausted his legal appeals when a federal judge found he didn't clear his name during an extraordinary innocence hearing and the Supreme Court refused to weigh in on that decision.

Davis' attorneys plan to ask the Georgia Board of Pardons and Paroles for clemency. The five-member panel, which will meet Sept. 19, has the power to commute or postpone executions but rarely does. The board halted his execution in 2007 but then refused to grant him clemency in 2008.

"We've been looking for other legal remedies, but it's a little unclear," said Jason Ewart, one of Davis' attorneys. "It's likely going to be in the hands of the pardons board. Any other remedy has likely run its course. But that's not to say there's not some glimmer of an option out there."

Davis has long claimed he's innocent of killing MacPhail, who was working off-duty at a Savannah bus station when he was shot twice. MacPhail was rushing to help a homeless man who prosecutors said was attacked by Davis. They say MacPhail pursued Davis, who fled and then turned around and shot the officer.

Eyewitnesses identified Davis as the shooter at his trial, but no physical evidence tied him to the slaying. He was convicted and sentenced to death in 1991.

Since then, his lawyers said new evidence has emerged that proves their client was a victim of mistaken identity. Several witnesses at his trial have recanted their testimony, and others who didn't testify have said another man who testified against Davis later confessed to the killing.

The Supreme Court in 2009 granted him an innocence hearing, the first time in at least 50 years the court had granted an American death row inmate such a hearing. It was a momentous decision because federal death penalty appeals normally look only at questions of due process and constitutional violations, not guilt or innocence.

During two days of testimony in June 2010, 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.

Moore said the evidence casts some additional doubt on Davis' conviction, but that it was "largely smoke and mirrors" and not enough to vindicate Davis. The 11th Circuit Court of Appeals and later the Supreme Court refused to hear his challenges.

The new execution date spurred Davis' supporters to plan rallies and petitions.

Laura Moye of Amnesty International USA said her group will stage several events for Davis and urge members to send letters on his behalf to Georgia's pardons board. The Rev. Al Sharpton asked supporters on Wednesday to do the same.

Benjamin Todd, president of the National Association for the Advancement of Colored People, urged Georgia to cancel what he called "the state-sanctioned murder of an innocent man."

"There's a big coalition of people from around the world who are watching," said Martina Correia, Davis' sister. "And they will fight for Troy."