Troy Davis' lawyers filing execution-day appeal

Updated at 10:21 a.m. ET

ATLANTA - Attorneys for Georgia death row inmate Troy Davis are filing a last-minute appeal to halt his execution later in the day.

Defense attorney Brian Kammer tells The Associated Press he will file the appeal in Butts County Superior Court, south of Atlanta, when it opens later Wednesday.

The appeal asks a judge to block the execution. It argues that ballistic testing that linked Davis to the shooting was flawed.

Davis is set to die at 7 p.m. EDT on Wednesday for the killing of off-duty Savannah officer Mark MacPhail, who was slain while rushing to help a homeless man being attacked.

Also Wednesday, attorney Stephen Marsh told The Associated Press that the Georgia Department of Corrections has denied his request to allow Davis to take a polygraph test. Marsh had said he hoped the polygraph test would convince the state pardons board to reconsider a decision against clemency.

Davis' lawyers have long argued Davis was a victim of mistaken identity. Prosecutors say they have no doubt that they charged the right person with the crime.

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Vigils outside Georgia's death chamber were set and protests were planned for Wednesday.

In Europe, where the planned execution has drawn widespread criticism, legislators and activists were making a last-minute appeal to the state of Georgia to refrain from executing Davis. Amnesty International and other groups planned a protest outside the U.S. Embassy in Paris later Wednesday. Amnesty International also planned to hold a vigil outside the U.S. Embassy in London on Wednesday night.

Renate Wohlwend of the Council of Europe's Parliamentary Assembly noted doubts raised about Davis' conviction by his supporters. She said that "to carry out this irrevocable act now would be a terrible mistake which could lead to a tragic injustice."

(At left, watch a "CBS Evening News" report on efforts to exonerate Davis)

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.

The U.S. Supreme Court gave him an unusual opportunity to prove his innocence last year, but his attorneys failed to convince a judge he didn't do it. State and federal courts have repeatedly upheld his conviction.

MacPhail's family lobbied the pardons board Monday to reject Davis' clemency appeal. The board refused to stop the execution a day later.

"He has had ample time to prove his innocence," said MacPhail's widow, Joan MacPhail-Harris. "And he is not innocent."

Spencer Lawton, the district attorney who secured Davis' conviction in 1991, said he was embarrassed for the judicial system that the execution has taken so long.

"What we have had is a manufactured appearance of doubt which has taken on the quality of legitimate doubt itself. And all of it is exquisitely unfair," said Lawton, who retired as Chatham County's head prosecutor in 2008. "The good news is we live in a civilized society where questions like this are decided based on fact in open and transparent courts of law, and not on street corners."

Davis supporters said they will push the pardons board to reconsider his case. They also asked Savannah prosecutors to block the execution, although Chatham County District Attorney Larry Chisolm said in a statement he was powerless to withdraw an execution order for Davis issued by a state Superior Court judge.

"We appreciate the outpouring of interest in this case; however, this matter is beyond our control," Chisolm said.

MacPhail was shot to death Aug. 19, 1989, after coming to the aid of Larry Young, a homeless man who was pistol-whipped in a Burger King parking lot. Prosecutors say Davis was with another man who was demanding that Young give him a beer when Davis pulled out a handgun and bashed Young with it. When MacPhail arrived to help, they say Davis had a smirk on his face as he shot the officer to death.

Witnesses placed Davis at the crime scene and identified him as the shooter. Shell casings were linked to an earlier shooting that Davis was convicted of. There was no other physical evidence. No blood or DNA tied Davis to the crime and the weapon was never found.

Davis' attorneys say seven of nine key witnesses who testified at his trial have disputed all or parts of their testimony.The state initially planned to execute him in July 2007 but the pardons board granted him a stay less than 24 hours before he was to die. The U.S. Supreme Court stepped in a year later and halted the lethal injection two hours before he was to be executed. And a federal appeals court halted another planned execution a few months later.

Over the years, Davis has picked up high-profile support from a host of dignitaries and dozens of federal lawmakers. Conservative figures have also advocated on his behalf, including former U.S. Rep. Bob Barr, ex-Justice Department official Larry Thompson and one-time FBI Director William Sessions.

The U.S. Supreme Court granted Davis a hearing to prove his innocence, the first time it had done so for a death row inmate in at least 50 years. At that June 2010 hearing, two witnesses testified that they falsely incriminated Davis at his trial when they said Davis confessed to the killing. Two others told the judge the man with Davis that night later said he shot MacPhail.

Prosecutors, though, argued that Davis' lawyers were simply rehashing old testimony that had already been rejected by a jury. And they said no trial court could ever consider the hearsay from the other witnesses who blamed the other man for the crime.

U.S. District Judge William T. Moore Jr. sided with them. He said the evidence presented at the hearing wasn't nearly enough to prove Davis is innocent and validate his request for a new trial. He said while Davis' "new evidence casts some additional, minimal doubt on his conviction, it is largely smoke and mirrors."