Lawyers at the US District Court in Riverside, east of Los Angeles, Tuesday began vetting a pool of around 120 jurors who will be whittled down to hear the case against Jose Nazario.
Nazario, 28, denies charges of voluntary manslaughter, assault with a dangerous weapon and discharging a firearm during a crime of violence in connection with the shootings of four Iraqi insurgents in 2004.
According to defense lawyers, Nazario's case is the first time a former soldier has been tried in a civilian court in connection with alleged crimes that took place during combat.
Prosecutors allege that Nazario, a former Marine Corps sergeant, shot dead two unarmed detainees on November 9, 2004 during a house search.
Nazario is also alleged to have ordered two Marines -- Ryan Weemer and Jermaine Nelson -- to shoot two other unarmed prisoners.
Weemer and Nelson have since been charged with unpremeditated murder and dereliction of duty and face courts martial later this year as they are still serving in the Marines.
Opening statements in Nazario's trial are expected on Thursday.
The case came to light after Weemer, 25, underwent a background screening for a job in the US Secret Service in 2006.
Asked if he had ever taken part in an unjustified killing, Weemer told his interviewer: "That actually did happen, to be honest."
The revelation triggered an investigation by the US Naval Criminal Intelligence Service which saw Nazario's squad mates questioned.
According to prosecutors' documents, Nazario and Marines belonging to K Company, 3rd Battalion, had radioed commanding officers to inform that they had captured four insurgents during a house search.
After being informed of the capture, Nazario was apparently asked by an unidentified senior officer: "Are they dead yet?"
The prisoners were subsequently shot, execution style, and the Marines left the house, according to prosecutors.
Lawyers for Nazario have said the case will set a dangerous precedent by allowing jurors with no military background to pass judgment on decisions taken during the heat of battle.
If Nazario is convicted, it could lead to hesitation amongst troops, putting themselves and fellow soldiers at risk, defense lawyers say.
"To second-guess the mind-set of a young man in the heat of battle four years later, and to put the question in a (civilian court) system that can't even remotely comprehend the battlefield, is shameful for this government," defense lawyer Joseph Preis told the North County Times.
"While not acknowledging that any part of the government's case is true, the fact is, the military sent these people out to do a job and kill everything that moved in a city where there were no good guys left," Preis added.
However prosecutors Jerry Behnke and Charles Kovats said in a pre-trial document that Marines had clear instructions how to handle prisoners.
"The killings were unlawful because they violated clearly established law of war," the prosecutors said. "All Marines, including the defendant, were repeatedly taught that they shall do no harm to detainees."
© 2008 Agence France Presse