LOS ANGELES: Nearly three years after the battle of Falluja earned the U.S. Marines more Navy Cross medals for heroism than any other action in Iraq, prosecutors are investigating whether members of one squad killed a group of captured insurgents there.

However, getting charges to stick could prove difficult as prosecutors try to assemble concrete evidence from a battle that reduced much of the city to rubble and caused extensive casualties. The identities of the victims are unknown.

Several marines are under investigation, and the former squad leader, now a civilian, has been charged in U.S. court with two counts of voluntary manslaughter.

About 130 marines were killed during the 53-day battle, more than 1,000 were wounded and about 1,000 insurgents were killed, said a Marine Corps spokesman, Lieutenant Colonel Chris Hughes. There is no count of civilian deaths.

"It's a little bit difficult to take a firefight three years after the fight and try to piece together whether or not a crime took place," said Doug Applegate, an attorney for Jose Nazario Jr., the former squad leader. "No crime scene could have been preserved, there's no physical evidence or DNA."

Nazario, 27, who has left the Marine Corps, pleaded not guilty last month.

Recent cases against marines over actions in Iraq highlight the challenges prosecutors face. Charges against eight marines in the killing of an Iraqi man last year in Hamdania resulted in only one murder conviction, despite confessions and testimony from several of the defendants. And prosecutors have yet to score any convictions against marines accused in the killings of 24 civilians in Haditha.

Observers say it will be even tougher to prosecute squad members in the Falluja case.

There are no forensics, the building where the shootings supposedly took place was destroyed and the identity of the victims is unknown, lawyers for some of the squad members said. Prosecutors identify the men Nazario is accused of shooting only as "human beings" called John Doe No. 1 and John Doe No. 2.

The investigation was triggered when a former corporal from the squad, Ryan Weemer, applied for a job with the Secret Service. Investigators claim he described the killings during a polygraph test that included a question about whether he had participated in a wrongful death, according to his attorney, Paul Hackett. Weemer has not been charged with any crime.

The complaint against Nazario says that after coming under fire from a house in Falluja, the squad entered the building and captured several insurgents, and Nazario placed a call on his radio.

"Nazario said that he was asked 'Are they dead yet?' " the complaint says. When Nazario responded that the captives were alive, he was told by a marine on the radio to "make it happen," the complaint says.

Applegate has said that investigators were looking into the actions of the marine who allegedly spoke with Nazario on the radio.

Lawyers say it is highly unusual for civilian prosecutors to go after a former U.S. serviceman for an alleged war crime. Kevin McDermott, another of Nazario's lawyers, said prosecutors employed a little-used 2000 law written primarily to prosecute civilian contractors who commit crimes while working for the U.S. overseas.

McDermott said he knew of only one other veteran, former Army Private Steven Green, who is charged in civilian court. Green is accused of raping and murdering a 14-year-old Iraqi girl and killing members of her family. He faces trial in Kentucky, and if convicted could get a death sentence.

Marine, army and Iraqi units entered Falluja on Nov. 9, 2004, and faced some of the heaviest fighting in the war in Iraq, often engaging in hand-to-hand combat.

Of more than 20 Navy Cross medals awarded for combat heroism in Iraq and Afghanistan, at least eight were earned in Falluja, according to several online sources. A Navy Cross is second only to a Medal of Honor.

Weemer's attorney, Hackett, a Marine reserve major, says it is unlikely that anyone who has never seen combat could grasp what marines experienced in Falluja.

"I remember the first day seeing a dog run down the street with an arm in its mouth. Dogs, cats eating bodies. Those are the kinds of scenes that a marine is experiencing," Hackett said.

"You take a 22-year-old American, you shoot at him all day long, you deprive him of sleep, you make him see his buddies being killed, he has their blood on his boots and blouse, and when you don't see perfection in his decisions you court-martial him? It's absurd."