Updated

The Navy is investigating claims that Camp Pendleton Marines killed between five and 10 unarmed suspected Iraqi insurgents who had been captured during a fierce battle in Fallujah in November 2004.

The criminal probe centers on the actions of several members of Kilo Company, 3rd Battalion, 1st Marines, current and former Marines told The Associated Press on condition of anonymity because they were not authorized to discuss the investigation.

Different members of the same unit were later accused of wrongdoing in the killings of 24 civilians in Haditha in 2005.

The investigation was launched when Ryan Weemer, a former Marine corporal injured while fighting in Fallujah, applied for a job with the Secret Service. When asked during a polygraph test if he had ever participated in a wrongful death, Weemer described the killings.

Weemer, 24, from Hindsboro, Ill., could not be reached for comment, but his sister Felicia Hudson said he was trying to put the event behind him.

"He does not like to talk about it," Hudson said. "He is very proud to be a Marine but he wants to get past all this and look to the future."

The Naval Criminal Investigative Service has confirmed it is investigating "credible allegations of wrongdoing made against U.S. Marines" in Fallujah in the fall of 2004.

News that investigators are looking into the actions of Camp Pendleton Marines was first reported last week by the North (San Diego) County Times.

Military author Nathaniel Helms this week posted a story online describing how he met Weemer last year while researching a book about the ferocious battles to recapture Fallujah from insurgents.

Weemer, who worked at Starbucks, told him that Marines killed several suspected insurgents who were being held in an abandoned house after they were captured in combat about Nov. 10, 2004.

The Marines radioed headquarters for guidance on how to proceed. "They're still alive?" came the response.

The leader interpreted the reply as an order to kill, Helms said. The captives were shot.

David Glazier, who teaches the law of war at Loyola Law School in Los Angeles, said it is a war crime to kill captives that do not pose an immediate threat unless they are escaping.

"Someone who has been taken into custody, they become protected under the law of war, no matter how egregiously they have behaved," Glazier said. "They can only be shot subject to the sentence of a validly conducted trial."

Helms said he warned a "penitent" Weemer to keep quiet about what he'd seen and did not mention the incident in his book, "My men are my heroes: The Brad Kasal story," named after a Marine sergeant who was praised for his combat actions in Fallujah.

After the Secret Service reported the incident to the military, NCIS investigators also contacted Helms. Hudson said her brother, who is now studying psychology in Kentucky, had not yet landed a job with the Secret Service.

A defense lawyer and former Marine captain who fought in Fallujah in December 2004 said the government would have a near-impossible task if it decided to prosecute the case.

"This is a huge rabbit hole and I can't see it going anywhere," attorney Brian Rooney said. "I was in Fallujah, it was nearly destroyed. The house is either gone or rebuilt completely, the bodies of the alleged victims are gone. Forensically you have no evidence."

Rooney, who represents Lt. Col. Jeffrey Chessani, one of four officers accused of failing to investigate the deaths of the 24 civilians in Haditha, said prosecutors would have to rely on eyewitness testimony to corroborate Weemer's account. With many of the Marines present at the time of the alleged killings already out of the Marine Corps, that could prove difficult.

Weemer's sister said her brother told Helms about the killings because he wanted people at home to understand the difficult conditions Marines faced in Iraq.

"His goal was to let people know what it was like over there, not for it to lead to this," she said.

Fallujah was the scene of two Marine battles in 2004, the first of which was launched after insurgents killed four U.S. contractors there. That battle was aborted in April 2004.

In November that year, Marines led an offensive against insurgent holdouts in the city, a fight that produced heavy casualties on both sides.

Camp Pendleton Marines already are the focus of two high-profile criminal cases, including the Haditha incident in which three enlisted Marines are charged with murder, and four officers are charged with failing to investigate the deaths. The Marines say they are not guilty because the deaths were the result of a lawful combat operation.

The other case centers on the actions of a different squad, charged with kidnapping and murdering an Iraqi man in Hamdania in April 2006. Five of the eight troops charged pleaded guilty to reduced charges; trials for the remaining three are due to begin next week.

Fallujah, Hamdania and Haditha are all in Al Anbar province, west of Baghdad.