ATLANTA — Georgia state officials have refused to spare the life of a Vietnam veteran seeking 11th-hour clemency who is scheduled to be executed Tuesday for murdering a sheriff's deputy during a traffic stop in 1998.

Andrew Brannan's lawyers argued before the State Board of Pardons and Paroles Monday that the 66-year-old combat veteran was suffering severe post-traumatic stress disorder and in the throes of an emotional flashback when he pulled a rifle from his pickup truck and exchanged gunshots with Lauren County Deputy Kyle Dinkheller 16 years ago.

The board voted later Monday to deny clemency. Brannan is to be executed with a massive overdose of the barbiturate pentobarbital at 7 p.m. Tuesday at the Georgia Diagnostic and Classification Prison in Jackson, Ga.

The Georgia State Parole Board listened to both sides in the case on Monday.

The five members of the parole and pardons board had the authority to commute or lessen the sentence against Brannan. Attorneys for Brannan spent 3 1/2 hours laying out their case for clemency, asking that their client's sentence be commuted to life without parole instead of the death penalty.

Lt. Andrew Brannan during the Vietnam War.

Photo Credit: Handout image

× Fear of missing out? Sign up for the Early Bird Brief - a daily roundup of military and defense news stories from around the globe. Thanks for signing up. By giving us your email, you are opting in to the Early Bird Brief.

In order to commute or reduce a sentence, three of the five board members must agree. Since 2002, the parole board has commuted five death sentences.

Dinkheller's murder after he stopped Brannan for speeding was captured on a video camera mounted on the officer's patrol car. In the 1998 dashcam, the veteran can be seen unloading his rifle at the deputy, shooting the husband and father nine times, killing him.

Dinkheller was 22 and his wife was pregnant. A jury later rejected a defense plea that Brannan was innocent by reason of insanity because of the PTSD he was suffering after serving as a forward artillery observer during intense combat in Vietnam. He had been rated 100% disabled by the Department of Veterans Affairs and had no prior criminal history.

Brannan's lawyers, who said the veteran was off of his bipolar medication at the time of the killing — have filed a last-minute legal appeal arguing that executing a mentally ill inmate violates the state and federal constitutions.

Brannan's attorney, Joe Loveland, released a statement regarding the parole board's decision.

"On behalf of Andrew Brannan and his family, we are profoundly disappointed in the decision of the Board of Pardons and Paroles to deny clemency. The death of Deputy Sheriff Kyle Dinkheller was a terrible tragedy. Executing a 66-year-old decorated Vietnam veteran with no prior criminal record who was seriously ill at the time of the crime only compounds the tragedy."

Brannan's attorneys have also filed for a writ of habeas corpus and a motion for a stay of execution in Butts County, where he is currently incarcerated. The filing would only buy the defense time to have the case re-examined.

Contributing: Owen Ullman, USA TODAY