* Soldier faces 16 counts of premeditated murder
* Shootings damaged U.S.-Afghan relations
* Army prosecutors seeking death penalty
A U.S. soldier accused of killing 16 villagers in Afghanistan is due to be arraigned on Thursday on charges of premeditated murder, for which military prosecutors are seeking the death penalty.
Robert Bales, a veteran of four combat tours in Iraq and Afghanistan, is accused of gunning down the villagers - mostly women and children - in their homes in two villages in Afghanistan's Kandahar province.
The shootings occurred over a five-hour period in March. It was one of the deadliest incidents the military has blamed on a rogue U.S. soldier since the Vietnam War and strained U.S.-Afghan relations.
Prosecutors say Bales, 39, acted alone and with "chilling premeditation" when, armed with a pistol, a rifle and a grenade launcher, he left his base twice, returning in the middle of his rampage to tell a fellow soldier: "I just shot up some people."
During a pre-trial hearing in November at Washington state's Joint Base Lewis-McChord, where Bales is being held and where Thursday's arraignment will take place, witnesses testified that he had been angered by a bomb blast near his outpost that severed a fellow soldier's leg days before the shootings.
The government believes Bales was solely responsible for the deaths, and survivors have testified that they saw only one U.S. soldier. However, several indirect accounts have suggested more than one soldier may have been involved.
Bales was bound over for court martial in December and faces 16 murder charges, as well as other charges, including attempted murder, assault and drug and alcohol charges.
"(The arraignment) makes it official, that he is about to go into a court martial. It also gives the defense and prosecution an opportunity to present motions," Army spokesman Gary Dangerfield said, adding that Bales would hear the summary of charges against him and be able to enter a plea.
Bales' lawyers declined requests for comment.
POSSIBLE POST-TRAUMATIC STRESS
Defense lawyers have not set out an alternate theory of the case, but have pointed out inconsistencies in pre-trial testimony and highlighted incidents before the shooting where Bales lost his temper easily.
Those incidents could possibly set up an argument that Bales was suffering from post-traumatic stress disorder.
"His defense attorneys would be wise to ask for further expert evaluation of his sanity," said Mary D. Fan, a former University of Washington law professor who is following the case. "They might be able to argue (that), at the time, Bales couldn't tell right from wrong."
Fan added that she expected Bales to enter a plea of not guilty or perhaps defer a plea pending further negotiation with prosecutors and an independent mental evaluation.
Army prosecutors told Reuters in November they would arrange for Afghan witnesses to testify in person during the court martial. Prosecutors have already presented physical evidence to link Bales to the crime scene, as well as blood-splattered camouflage clothing.
For Bales to be executed, a military jury must unanimously find him guilty of the eligible crime and at least one aggravating factor that largely outweighs any mitigating circumstances, the Army said.
Evidence of any mental illness, post-traumatic stress disorder, extended combat experience, and the presence of alcohol and other substances could be "critical evidence," said civilian defense attorney Colby Vokey, a retired Marine lieutenant-colonel.
The military justice system also requires the U.S. president to approve the execution of a service member, which last occurred in 1961.
"You can't receive the death penalty in the military with anything less than premeditation," said Victor Hansen, vice-president of the National Institute of Military Justice.