A former Marine accused of killing unarmed Iraqi detainees was acquitted of voluntary manslaughter Thursday in a first-of-its-kind federal trial.

The jury took six hours to find Jose Luis Nazario Jr. not guilty of charges that he killed or caused others to kill four unarmed detainees on Nov. 9, 2004, in Fallujah, Iraq, during some of the fiercest fighting of the war.

The verdict left the 28-year-old defendant in tears. He cried so loud that the judge smacked his gavel to call for order. Nazario's family and friends also sobbed in the courtroom.

"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."

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

One of the jurors, Ingrid Wicken, hugged Nazario's sobbing mother, Sandra Montanez, without speaking after the verdict was read. "I watched her all week. She was being tortured every day," Wicken said later.

Wicken said the panel acquitted Nazario because there was not enough evidence against him.

"I think you don't know what goes on in combat until you are in combat," she said.

Nazario's attorney, Kevin McDermott, said he believes the verdict will curb faulty filings.

"I don't think they are going to put on a case in the future with a lack of evidence," McDermott said.

Prosecutors alleged that Nazario either killed or caused others to kill four unarmed Iraqi detainees in Fallujah during "Operation Phantom Fury," which resulted in house-to-house fighting.

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

The case came to light in 2006 when Sgt. Ryan Weemer, Nazario's former squadmate, volunteered details to a U.S. Secret Service job interviewer during a lie-detector screening that included a question about the most serious crime he ever committed. That screening was not admitted at Nazario's trial.

Weemer and another Marine, Sgt. Jermaine Nelson, face military charges of unpremeditated murder and dereliction of duty. Both maintain their innocence, and both were found in contempt of court for refusing to testify against Nazario.

Had Nazario been convicted of voluntary manslaughter, assault with a deadly weapon and discharging a firearm during a crime of violence, he could have faced more than 10 years in prison.

On Wednesday, federal prosecutor Jerry Behnke urged the jury to convict Nazario, saying he violated his duty as a Marine and must be held accountable for his actions in Fallujah. He said the evidence showed the detainees had surrendered before the shooting.

McDermott told jurors they could not convict the former Marine sergeant of an alleged crime in which there were no bodies, no identities and no forensics. He also argued that a guilty verdict would only make service members second-guess their actions in combat.

Nazario is the first former military service member brought to trial under the Military Extraterritorial Jurisdiction Act, which was written in 2000 and amended in 2004 primarily to allow prosecution of civilian contractors who commit crimes while working for the U.S. overseas. It also allows the prosecution of military dependents and former military service members accused of committing crimes outside the United States.