A former US marine has been acquitted of voluntary manslaughter in the killing of unarmed detainees in Iraq four years ago.

The acquittal of Jose Luis Nazario Jr came after six hours of jury deliberations in the first-of-its-kind federal trial in a California court on Thursday.

Nazario was charged with killing or causing others to kill four unarmed detainees in Falluja during some of the fiercest fighting of the war in Iraq in 2004 which saw house-to-house fighting.

Other former marines testified during the five-day trial that they heard gunshots but did not see Nazario kill the detainees.

Thursday's verdict marks the first time a civilian jury has determined whether the actions of a former soldier in combat violated the law of war.

"It's been a long, hard year for my family," Nazario said outside the courtroom.

"I need a moment to catch my breath and try to get my life back together."

Ingrid Wicken, the jury forewoman, said the panel acquitted Nazario because of insufficient evidence.

"I think you don't know what goes on in combat until you are in combat," she said.
 
The former marine's lawyer, Kevin McDermott, said he believed the verdict would curb faulty legal filings.
 
"I don't think they are going to put on a case in the future with a lack of evidence."