The incident took place in November 2005
A US marine lied to cover up a squad's killings of 24 civilians in Iraq's city of Haditha in 2005, a US prosecutor has said at the officer's court martial.
The prosecutor's comments came during opening statements in the trial of Lt Andrew Grayson in California.
Lt Grayson is charged with obstructing justice and making false statements in connection with the case. He rejects the allegations.
He is the first of three defendants to go on trial.
Four marines were initially charged with killing of the 24 unarmed Iraqi civilians, including women and children, in Haditha. Another four were charged with failing to investigate the deaths.
But five of the marines have now had charges dismissed.
Lt Grayson lied to investigators to help cover up the killings in Haditha, prosecutor Lt Col Paul Atterbury told a seven-member jury at the court martial in Camp Pendleton.
The dead in Haditha included women, children and elderly men
The prosecutor also said there were key undisputed facts in the case: a roadside bomb on 19 November 2005 killed a marine and injured two others, and two hours later the squad's actions resulted in the deaths of the 24 Iraqis.
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.
He is charged with six counts of making false statements and obstructing justice.
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.
If found guilty, he could face up to 30 years in prison, forfeiture of all pay and dismissal from the Marine Corps.
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.