The charges relate to events in Falluja in 2004
A US marine has gone on trial accused of killing an unarmed captive in Iraq, in a case sparked by his own alleged confession during a lie-detector test.
The court-martial in California heard that when Sgt Ryan Weemer applied for a Secret Service job, he said he had killed someone in Iraq in 2004.
That led to a criminal investigation and he was recalled to active duty to face military prosecution.
He denies the charge and his defence argues the case cannot be proved.
At the court-martial, being held at Camp Pendleton marine base near San Diego, his lawyer told the jury there was no evidence a crime had ever been committed.
He said prosecutors could not prove their case because they had no body, no forensic evidence and no relatives complaining of a lost loved one.
But prosecutors have at least one recording of Sgt Weemer saying he shot a man.
He is charged with unpremeditated murder and dereliction of duty stemming from a fierce fire-fight in Falluja in 2004.
Sgt Weemer left the Marine Corps two years later and applied for a job in the Secret Service.
During a polygraph test as part of the application, he was asked about the most serious crime he had ever committed.
"We went into this house, there happened to be four or five guys in the house," he said on the recording, which was played at the trial.
"We ended up shooting them, we had to."
The US military had ordered all civilians out of Falluja ahead of an assault - Operation Phantom Fury - aimed at recapturing the city from insurgents.
Sgt Weemer said in the interview that the unarmed Iraqis were killed because the marines did not have time to take the men to jail.
His squadron leader, Jose Nazario, has already been tried and was found not guilty.
A third marine, who is also charged with murder, faces trial in a few weeks.