The incident took place in November 2005
The first US court martial to arise from the 2005 killing of 24 Iraqi civilians in the northern Iraqi city of Haditha has begun in California.
Lt Andrew Grayson is charged with obstructing justice and making false statements in connection with the case. He rejects the allegations.
Prosecutors accuse US marines of killing 24 unarmed Iraqi civilians, including women and children.
Five of the eight men initially accused have now had charges dismissed.
Lt Grayson would also have had his charges dropped if he had accepted a plea deal and admitted that he covered up the killings, his lawyer said.
The court martial began in Camp Pendleton, California, with jury selection from a pool of officers and enlisted personnel.
They were then briefed in court on the case against Lt Grayson, who is charged with six counts of making false statements and obstructing justice.
If found guilty, he could face up to 30 years in prison, forfeiture of all pay and dismissal from the Marine Corps.
Lt Grayson is the first of the three defendants to go on trial.
The case stems from a roadside bombing in Haditha on 19 November 2005 which killed one marine, L/Cpl Miguel Terrazas, and injured two others.
The dead in Haditha included women, children and elderly men
The US military at first reported that the Iraqis had been killed by that explosion, or in a subsequent gunfight with insurgents.
But Iraqi witnesses said the US troops shot dead five unarmed men when they approached the scene of the bombing in a car.
The troops were then accused of killing 19 other civilians in three houses nearby over the next few hours.
Lt Grayson is alleged to have instructed a sergeant to delete digital photographs of the incident from his camera.
Despite the accusations, there was no full US investigation into what happened until January 2006, when video footage emerged of the aftermath, filmed by a local human rights activist.
A subsequent investigation by Time magazine suggested that most of the dead were shot by marines after the bombing - and in March 2006 a criminal investigation was launched.
The following month, three officers in charge of troops in Haditha were stripped of their command and reassigned.
Those accused of involvement in the incident have maintained they were reacting to insurgent fire.
One of the men, Staff Sgt Frank Wuterich, faces a court martial on nine counts of voluntary manslaughter later this year.
The third marine to face a court martial will be battalion commander Lt Col Jeffrey Chessani, who has been charged with dereliction of duty and violation of a lawful order.