(CNN) -- A former Marine sergeant accused of killing two Iraqi captives in a 2004 battle in Iraq said his squad fabricated the entire story.
Former Marine Sgt. Jose Luis Nazario faces manslaughter charges.
Jose Luis Nazario has pleaded not guilty to voluntary manslaughter charges brought by federal prosecutors in southern California. He left the service in 2005.
According to an affidavit from a Navy investigator, Nazario killed one prisoner and then asked his squad: "Who else wants to kill these guys? Because I don't want to do it all myself."
The prisoners -- four men who had been captured in a house that was the source of hostile gunfire -- were killed so the Marines could keep up with the rest of their unit in the early days of the November 2004 battle to recapture Falluja, the affidavit states.
But Kevin McDermott, Nazario's attorney, said his client denies the allegations completely.
"They didn't happen," McDermott told CNN.
He called the charges "a fish story" that originated from a former member of Nazario's squad who first made the allegations during a 2006 polygraph test the squad members took when applying for a job with the Secret Service.
McDermott said military investigators appear to be trying to find out who is supposed to have given Nazario an order to kill the prisoners -- a radio call recounted in the affidavit. The document quotes interviews with three other Marines addressing the purported call.
"I think the government believes the story, and I think they're interested in finding out who was on the other end of the line," McDermott said.
But, the attorney insists there was no such call.
"That call never occurred," McDermott said. "That's what's so crazy about this whole situation."
Nazario was allowed to remain free on $50,000 bail after his arraignment last week in Riverside, California. He had taken a job as a police officer after leaving the Marines.
Nazario was still within his probationary period and was fired after his arrest, McDermott said.
McDermott said his client passed two polygraph tests before joining the police force and was specifically asked during those sessions whether he had violated the laws of warfare during his military service.
Prosecutors say the killings took place November 9, 2004 -- the second day of the campaign to retake Falluja, a restive Sunni Arab city west of Baghdad that had fallen under insurgent control in spring 2004.
U.S. troops faced fierce house-to-house fighting during a battle that lasted more than a week. A member of Nazario's squad had been killed earlier in the day.
The squad had captured several men inside a house that had been the source of hostile gunfire, members told Naval Criminal Investigation Service agent Mark Fox. When Nazario reported the capture, he told his fellow Marines, he was asked "Are they dead yet?"
When Nazario said no, he told his squad mates that he was instructed to, "Make it happen," according to the affidavit.
Once off the radio, Nazario said, "We can't be here all day. You know what has to be done," the affidavit states.
Another member of the squad, Sgt. Jermaine Nelson, has been charged with murder. Nelson is still in the Marines and based at Camp Pendleton, California. He remains free while awaiting a hearing, a base spokesman said Monday.
McDermott said investigators have no forensic evidence and can't identify the men the squad is supposed to have executed.
"I really think this is a play to identify who the officer is that they talked to," he said.
A third Marine, who has not been charged, shot another prisoner, according to the affidavit. Prosecutors blacked out the names of the witnesses before releasing a copy to CNN.
The conduct of U.S. troops during the battle for Falluja came under scrutiny at the time when a television reporter recorded the shooting of four apparently wounded men in a mosque compound.
Military investigators later determined the Marine corporal who pulled the trigger acted in self-defense, "believing they posed a threat to him and his fellow Marines." E-mail to a friend