Nine witnesses testified against Mr. Davis at his trial in 1991, but seven of the nine have since changed their stories. One of the recanting witnesses, Dorothy Ferrell, said she was on parole when she testified and was afraid that she’d be sent back to prison if she didn’t agree to finger Mr. Davis.

Image Bob Herbert

She said in an affidavit: “I told the detective that Troy Davis was the shooter, even though the truth was that I didn’t know who shot the officer.”

Another witness, Darrell Collins, a teenager at the time of the murder, said the police had “scared” him into falsely testifying by threatening to charge him as an accessory to the crime. He said they told him that he might never get out of prison.

“I didn’t want to go to jail because I didn’t do nothing wrong,” he said.

At least three witnesses who testified against Mr. Davis (and a number of others who were not part of the trial) have since said that a man named Sylvester “Redd” Coles admitted that he was the one who had killed the officer.

Mr. Coles, who was at the scene, and who, according to authorities, later ditched a gun of the same caliber as the murder weapon, is one of the two witnesses who have not recanted.

The other is a man who initially told investigators that he could not identify the killer. Nearly two years later, at the trial, he testified that the killer was Mr. Davis.

So we have here a mess that is difficult, perhaps impossible, to sort through in a way that will yield reliable answers. (The jury also convicted Mr. Davis of a nonfatal shooting earlier that same evening on testimony that was even more dubious.)