Washington ? 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 an off-duty police officer almost 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 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.

But Davis’ lawyers say new evidence proves their client was a victim of mistaken identity. They say three people who did not testify at Davis’ trial have said another man confessed to the killing.

Davis’ attorneys have delayed his execution three times by raising doubts about those witnesses. But state and federal courts have denied Davis’ request for a new trial.