Jose Nazario denies allegations he shot dead unarmed Iraqi prisoners
The civil trial has opened of a former US marine accused of killing unarmed Iraqi detainees in the city of Falluja, in the first case of its kind.
Jose Luis Nazario, 28, denies charges of voluntary manslaughter and assault over the deaths of four Iraqis in 2004.
He is the first former US soldier no longer serving to be tried for crimes committed while on active duty.
The trial, in the state of California, will use a law designed to be used for prosecuting civilian contractors.
Mr Nazario, who left the US Marine Corps in 2006, could face up to 10 years in jail if found guilty of all the charges against him.
Jury selection for the trial started on Tuesday and opening statements are expected later this week.
Two active service marines allegedly involved in the same incident face courts martial later this year.
According to Navy documents, several marines allege that Mr Nazario shot dead two Iraqi men who had been detained while his squad searched a house.
He is alleged to have ordered two other marines to shoot two other unarmed prisoners.
Some legal experts say the case raises questions about how those accused of breaking the law during combat should be brought to justice afterwards, says the BBC's Rajesh Mirchandani in Los Angeles.
In an interview, Mr Nazario said he was being prosecuted for using the training he was given.
The alleged incident occurred during some of the fiercest fighting in Iraq, as US forces battled for control of Falluja in 2004.