ATLANTA -- Georgia's board of pardons rejected a last-ditch clemency bid from Troy Davis on Tuesday, one day before his scheduled execution, despite support from figures including a former president and a former FBI director for the claim that he was wrongly convicted of killing a police officer in 1989.

Davis is scheduled to die tonight at 7 by injection for killing off-duty Savannah officer Mark MacPhail, who was shot dead while rushing to help a homeless man being attacked.

It is the fourth time in four years that Davis' execution has been scheduled by Georgia officials.

"Justice was finally served for my father," said Mark MacPhail Jr., who was an infant when his father was gunned down. "The truth was finally heard."

Davis has captured worldwide attention after his supporters raised concerns that he was a victim of mistaken identity. Several witnesses who helped convict Davis at his 1991 trial have backed off their testimony or recanted.

Others who did not testify have said that another man at the scene admitted to the shooting.

Prosecutors said they have no doubt that they charged the right person with the crime, and MacPhail's family lobbied the pardons board on Monday to reject Davis' clemency appeal.

The decision appeared to leave Davis with little chance of avoiding his execution date. The state Board of Pardons and Paroles has sole authority in Georgia to commute an inmate's sentence; Gov. Nathan Deal has no say on the issue.

Defense attorney Jason Ewart didn't rule out filing another legal appeal.

MacPhail was shot to death Aug. 19, 1989, after coming to the aid of Larry Young, a homeless man who was pistol-whipped in a parking lot. Prosecutors said Davis was with another man who was demanding that Young give him a beer when Davis pulled out a handgun and bashed Young with it.

When MacPhail arrived to help, they said Davis shot the officer to death.

Among Davis' supporters are former President Jimmy Carter and former FBI Director William Sessions.