The Supreme Court on Monday ordered a new hearing for death row inmate Troy Davis, whose supporters say is innocent and should be spared from execution for killing a police officer 20 years ago.

Davis has spent 18 years on death row for the 1989 slaying of Savannah, Ga., police officer Mark MacPhail. Davis' attorneys insist that he is innocent and deserves a new trial because several witnesses at his trial have recanted their testimony.

The high court ordered a federal judge in Georgia to determine whether there is evidence "that could not have been obtained at the time of trial (that) clearly establishes petitioner's innocence."

Defense lawyers had appealed to the Supreme Court after a federal court denied a new trial request in April.

"The substantial risk of putting an innocent man to death clearly provides an adequate justification for holding an evidentiary hearing," said Justice John Paul Stevens, writing for the court. Justices Ruth Bader Ginsburg and Stephen Breyer concurred with Stevens.

MacPhail was slain 20 years ago while working off-duty as a security guard at a bus station. He had rushed to help a homeless man who had been pistol-whipped at a nearby parking lot, and was shot twice when he approached Davis and two other men. Witnesses identified Davis as the shooter at his 1991 trial.