ATLANTA -- Troy Davis is set to be executed Tuesday, over arguments that he's a victim of mistaken identity and hardball investigation tactics. Davis was convicted of killing a Savannah police officer 18 years ago. The state, which plans to give Davis a lethal injection in Jackson, said he received a fair trial and has had plenty of appeals that have failed. Monday is Davis's last, best hope at avoiding execution. His attorneys will go before the state board of Pardons and Paroles to argue his sentence should be commuted to life in prison. U.S. Representative John Lewis, an Atlanta Democrat and civil rights icon, plans to testify on Davis' behalf out of concern that the state is planning to execute an innocent man. Some 4,000 letters have been presented to the board on Davis' behalf -- including some from religious leaders, entertainers and civil and human rights leaders. A jury convicted Davis, 38, for the August 19, 1989, murder of Officer Mark MacPhail, who was shot twice after he rushed to help a homeless man who had been assaulted. The shooting took place in a Burger King parking lot next to the bus station where MacPhail worked off-duty as a security guard. Lawyers say seven of the nine witnesses who testified at his trial that they either saw Davis shoot the officer, saw him assault the homeless man before the shooting or heard Davis later confess to the slaying have since recanted or contradicted their testimony. Other affidavits from three people who did not testify at Davis' trial say another man confessed to killing the officer after Davis was convicted.