Barring intervention from the U.S. Supreme Court or Virginia's governor, a man convicted of killing a security guard during a bank robbery will be executed Thursday.

John Yancey Schmitt, 33, is set to die by injection at 9 p.m. ET for the fatal shooting of 39-year-old Earl Shelton Dunning.

Dunning, who had recently been hired to provide security, became suspicious of Schmitt when he saw him enter the bank wearing sunglasses and a bulky jacket on Feb. 17, 1999. Dunning followed him inside, where he was shot. Schmitt then fled with nearly $36,000 in cash. He and an accomplice had robbed the same bank a month earlier.

In 2000, a jury convicted Schmitt of capital murder and robbery and firearms charges and recommended a death sentence, declaring him a future danger to society.

In their petition to the U.S. Supreme Court and in a clemency petition to Gov. Timothy M. Kaine, Schmitt's attorneys accuse prosecutor Warren Von Schuch of withholding information about a key witness, which they say could have influenced the jury's decision to recommend a death sentence. In the clemency petition, they also argue the shooting of Dunning was accidental.

Von Schuch denied any prosecutorial misconduct, and called the killing premeditated and cold-blooded.

In July, a three-judge panel of the 4th U.S. Circuit Court of Appeals in Richmond criticized Von Schuch for withholding the information, but ultimately ruled the prosecutor's actions would probably not have impacted the jury's decision.

In their clemency petition to Kaine, Schmitt's attorneys also argue forensic evidence shows that Schmitt and Dunning struggled and that Schmitt's gun fired accidentally. The bank's security cameras didn't record any images of a confrontation or the shooting.

Dunning, who had recently retired from the Army, was killed just a month before he planned to be married. Von Schuch said Dunning bravely sacrificed his own life by placing himself between Schmitt and the bank tellers.

"He was a tremendous human being," Von Schuch said. "This man really was a hero."