Troy Davis was denied clemency on Tuesday in a last-ditch appeal and is scheduled to be executed by lethal injection on Wednesday for the 1989 murder of a police officer in Savannah.

42-year-old Davis was originally convicted in 1991 and since has appealed four times before the Georgia State Board of Pardons and Paroles, the only authority in the state with the power to alter his sentence.

The case has roused much controversy since many of the witnesses in his original trial have changed their testimony. Davis has captured the support from many to include former President Jimmy Carter and Pope Benedict XVI and protests against the judicial killing for Davis, who has maintained his innocence.

Troy Davis is set to be executed by lethal injection at 7 p.m. on Wednesday evening.

Here is a timeline of events leading up to the execution of Troy Davis since 1989.

Aug. 19, 1989: 27-year-old security guard Mark MacPhail was shot dead in Savannah after attempting to help a homeless man named Larry Young being beaten by an attacker in a parking lot.

Aug. 23, 1989: 19-year-old Troy Davis is arrested and charged with murder after a man present on the scene, Sylvester Redd Cross. Cross pins Davis as the suspect, saying he was leaving the scene after Young was attacked, just before MacPhail was shot. Prosecutors said Davis hit Young with a gun and fired multiple shots at MacPhail.

Aug. 30 1991: The trial for Davis begins. While there was no physical evidence or murder weapon found, nine witnesses say Davis killed MacPhail. Davis pleads not guilty, saying he left the scene before the shooting took place after witnessing Coles hit the homeless man. Davis is found guilty and the jury sentences Davis the death penalty, placing him on death row.

Sept. 2003: Seven of the nine witnesses for Davis' trial recant their testimony, changing their stories after being pressured by the police. Their new testimony is published in the Atlanta Journal-Constitution, along with statements made by Coles saying he is the real killer. If I knew then what I know now, Brenda Davis, one of the jurors in the trial told CNN in a 2009 interview, Troy Davis would not be on Death Row. The verdict would be 'not guilty.'

July 16, 2007: Davis is granted a 90-day stay of execution to review the case, one day before his initial execution date.

Sept. 23, 2008: The U.S. Supreme Court delays the second execution date, sparing the life of Davis. The decision whether or not to hear the case was pending, after ordering a federal judge in Savannah to convene a hearing to consider new evidence.

Oct. 27, 2009: The U.S. Supreme Court elects not to hear the case. Though a third date was set, the Georgia appeals court puts a hold for the execution for a petition period.

June 2010: Two witnesses tell a U.S. District judge that they falsely incriminated Davis while two others said another man had confessed to killed MacPhail.

August 2010: A Georgia District Court rules Davis failed to prove his innocence and denied him a new trial.

Sept. 7, 2011: Georgia sets the fourth execution date for Sept. 21, 2011.

Sept. 15, 2011: Clemency is asked by 663,000 people who signed a petition, sent to the Georgia Board of Pardons and Paroles. Petitions from the public, Jimmy Carter and Beatrice King, daughter of Martin Luther King Jr. continue for days.

Sept. 19, 2011: The five-member Board of Pardons and paroles hear Davis' plea for clemency, two days before his fourth execution date. The board meets in a closed-door session and will hear testimony from Davis' attorney, prosecutors, MacPhail's family and witnesses.

Sept. 20, 2011: Davis is denied clemency from the Georgia Board of Pardons and paroles.

Sept. 21, 2011: An execution for Troy Davis is scheduled for 7 p.m. by lethal injection.