This post has been updated.

After 20 years, and four scheduled executions, the case of Troy Davis may come to an end on Wednesday.

Troy Davis was convicted nearly two decades ago and sentenced to die for the 1989 slaying of Savannah police officer Mark MacPhail. Several witnesses have since recanted testimony. (AP)

In one of the most high profile death row sentences, the Georgia Board of Pardons and Paroles returned a verdict on Tuesday morning: clemency for Davis is denied, the Atlanta Journal-Constitution reports.

It was his last legal appeal.

Wednesday is the fourth execution date scheduled for Davis, after decades of twists and turns in a murder case that has attracted controversy for years.

Davis, 42, is scheduled to be executed by lethal injection Wednesday evening.

In 1989, an off-duty Savannah police officer Mark MacPhail was shot and killed. Nine eyewitness identified Davis as the murderer, even though no phsyical evidence tied him to the crime.

In the years since, seven of the nine eyewitnesses recanted their story and two other witnesses said another man committed the crime. Amid the doubt, three previous execution dates have been postponed by court order, one just mere hours before Davis was set to die.

Last week, a petition with 663,000 signatures was delivered to the Georgia board, calling for clemency. Former president Jimmy Carter and Pope Benedict XVI have come forward asking for a stay of execution.

Songwriter Nellie McKay has also stepped forward to lend her support to the clemency case, writing an original song for Davis just days ago in the hopes it would draw attention to Davis.

McKay, who currently has a New York cabaret show based on the third woman executed by the state of California, became immersed in death row research when working on her show. She said it was an institutionalized form of violence -- the worst possible kind of violence in her mind.

With the help of a saxophone player and a videographer friend, McKay put together this haunting video sprinkled with facts about the case. In a phone interview she said, “There’s a cacophony on his side, and I hope the board listens.”