The Supreme Court on Monday ordered a new hearing for death row inmate Troy Davis, giving the condemned killer a chance to present evidence his lawyers say could clear him in the murder of an off-duty police officer almost 20 years ago.

The court told a federal judge to hold a hearing to decide whether evidence "that could not have been obtained at the time of trial" could establish Davis' innocence. His case has become a rallying cry for death penalty opponents.

Davis' attorneys contend he deserves a new trial because new evidence proves he was mistakenly identified as the killer. They say several trial witnesses have recanted their testimony, and others who did not testify during the trial have said another man confessed to the killing.

"This is exactly what we asked for," said Jason Ewart, Davis' attorney. "It's been years since these witnesses have come forward, and they've never had their day in court. And now they will."

Davis was convicted 18 years ago for the 1989 slaying of Savannah, Ga., police officer Mark MacPhail, who was shot twice while working off-duty as a security guard at a bus station. He was gunned down after rushing to the aid of a homeless man who had been attacked.

Davis' execution has been delayed three times, including a ruling by an appeals court last year that gave the 40-year-old another chance to press his appeal. The panel rejected the appeal in April, and federal and state courts have repeatedly denied his request for a new trial.