From Amnesty at the above link:

Troy Davis was sentenced to death for the murder of Police Officer Mark Allen McPhail at a Burger King in Savannah, Georgia; a murder he maintains he did not commit. There was no physical evidence against him and the weapon used in the crime was never found. The case against him consisted entirely of witness testimony which contained inconsistencies even at the time of the trial. Since then, all but two of the state's non-police witnesses from the trial have recanted or contradicted their testimony. Many of these witnesses have stated in sworn affidavits that they were pressured or coerced by police into testifying or signing statements against Troy Davis.

One of the two witnesses who has not recanted his testimony is Sylvester "Red" Coles – the principle alternative suspect, according to the defense, against whom there is new evidence implicating him as the gunman. Nine individuals have signed affidavits implicating Sylvester Coles.