(CBS/AP) ATLANTA – A defense attorney says Georgia prison officials have blocked inmate Troy Davis from taking a polygraph test before his scheduled execution.

Attorney Stephen Marsh told The Associated Press on Wednesday that the Georgia Department of Corrections has denied his request to allow Davis to take a polygraph test. Davis is scheduled to die at 7 p.m. EDT Wednesday.

Davis has long claimed he is innocent of killing Mark MacPhail, an off-duty police officer working as a security guard in Savannah, Ga. But state and federal courts have repeatedly upheld his conviction.

Prosecutors and MacPhail's relatives say they have no doubt the right man is being punished. Marsh had said he hoped the polygraph test would convince the state pardons board to reconsider a decision against clemency.

After winning three delays since 2007, Davis lost his most realistic chance at last-minute clemency this week when the state pardons board denied his request.

Some witnesses who fingered him at trial as the shooter later recanted, and others who did not testify came forward to say another man did it. But a federal judge dismissed those changed and new accounts as "largely smoke and mirrors" after a hearing Davis was granted last year to argue for a new trial, which he did not win.

Davis didn't want a last meal. He planned to spend his final hours meeting with friends, family and supporters. According to an advocate who met with him late Tuesday, he was upbeat, prayerful and expected last-minute wrangling by attorneys.

Davis has received support from hundreds of thousands of people, including a former FBI director, former President Carter and Pope Benedict XVI. Some of his backers resorted to urging prison workers to strike or call in sick Wednesday, and they considered a desperate appeal for White House intervention. And some of Davis' supporters were considering whether to ask President Obama to intervene, a move that legal experts said was unlikely.

Check out a special report below: