By Laura L. Myers

SEATTLE (Reuters) - A U.S. soldier charged with killing 16 civilians, most of them women and children, near his Army post in Afghanistan is set to undergo a medical review on Sunday to determine his state of mind at the time of the killings and ability to stand trial.

The review, known in the military as a "sanity board," will be conducted by three doctors at the Joint Base Lewis-McChord in Washington state, and will be completed by May 1, according to a U.S. Army spokesman.

Prosecutors are seeking the death penalty against Robert Bales, a decorated veteran of four combat tours in Iraq and Afghanistan who is accused of gunning down the villagers in cold blood during two rampages through their family compounds in Kandahar province last March.

Army prosecutors say Bales, a 39-year old father of two, acted alone and with "chilling premeditation" when, armed with a pistol, a rifle and a grenade launcher, he left his base twice in the night, returning in the middle of his rampage to tell a fellow soldier: "I just shot up some people."

The shootings marked the worst case of civilian slaughter blamed on a rogue U.S. soldier since the Vietnam War and further eroded strained U.S.-Afghan relations after more than a decade of conflict in that country.

Defense lawyers have not set out an alternative theory of what happened on the night of the shootings, but have focused on Bales' fragile mental state.

Bales' lead civilian attorney John Henry Browne said in January that government documents showed Bales had been diagnosed as suffering from post-traumatic stress disorder (PTSD) and a brain injury before his deployment in Afghanistan in 2011.

After hearing preliminary evidence in November, military judge Colonel Jeffery Nance determined that Bales should face a court martial, which is due to begin in September.

At Bales' arraignment in January, Browne's team entered no plea and told Nance they were preparing a possible "mental health defense." Nance said such a defense would require a formal psychiatric evaluation, and ordered a sanity board review.

SANITY BOARDS COMMON

Sanity boards are common in military justice, said Army spokesman Lieutenant Colonel Gary Dangerfield.

They may be ordered when a commander, investigating officer, government trial counsel, defense counsel, or military judge believes there is reason to question the mental responsibility of the accused at the time of the offenses or that the person lacks the mental capacity to stand trial, Dangerfield said.

The Army has not said which doctors will conduct the review, or what their specialties are, but such panels are usually made up of Army physicians and psychologists. There was no word on whether the judge had granted a defense request that Bales be examined by a neuropsychologist with expertise in traumatic brain injuries.

At a November pre-trial hearing, prosecutors said Bales had been drinking earlier in the evening of the attacks, and had used steroids on the Special Forces outpost. Defense attorneys also quizzed Bales' colleagues closely about his sudden explosions of temper in the days before the attacks.

Bales faces 16 counts of premeditated murder and six counts of attempted murder, as well as charges of assault and wrongfully possessing and using steroids and alcohol while deployed.

Military justice experts say a defense based on Bales' PTSD or deeper mental health problems may not be enough to avoid trial, but could raise serious issues over premeditation, which would make a death sentence less likely.

"Just because someone has a (mental) disease does not mean they're legally insane," said Victor Hansen, a professor at the New England Law Boston law school with two decades of military law experience.

"The board could uncover unknown components that could help the defendant claim diminished capacity," Hansen said. "To obtain death, you have to prove premeditation. Anything less than premeditation of murder, there's no death."

Defendants deemed unable to stand trial after a sanity board are typically referred for treatment at a government medical center.

(Writing by Bill Rigby; Editing by Cynthia Johnston and Lisa Shumaker)