The US Navy has confirmed that it is investigating "credible allegations" of wrongdoing by US marines in the Iraqi city of Falluja in 2004.
The inquiry centres on claims that members of a marine unit killed several unarmed Iraqi captives, according to the Associated Press news agency.
Different members of the same unit were later charged over the killings of 24 civilians in the Iraqi town of Haditha.
The navy has yet to confirm any details of the alleged wrongdoing in Falluja.
The investigation was launched after a former member of Kilo Company, 3rd Battalion, 1st Marines, based at Camp Pendleton, reportedly revealed details while taking a polygraph test for a Secret Service job.
Military author Nathaniel Helms, who has interviewed the former marine involved, has published an online report claiming the suspected insurgents were killed while being held captive in an abandoned house.
The US Navy Criminal Investigation Service (NCIS) said its findings would be presented to the relevant authorities, who would decide whether further action was warranted.
"NCIS can confirm that it is conducting an investigation into credible allegations of wrongdoing made against US Marines concerning actions said to have taken place in Falluja, Iraq, in the fall of 2004," it said in a statement.
Civilian lawyers have told the BBC that NCIS agents are actively pursuing the investigation and that, to their knowledge, it concerns allegations of murder in Falluja.
But, says the BBC's Adam Brookes in Washington, the lawyers agree that this would be a very hard case to prosecute because there would seem to be no forensic evidence, no crime scene, no bodies and no eyewitnesses other than the marines themselves.
The Haditha case involves the alleged killing of 24 civilians in the town of Haditha, in Anbar province, north-west of Baghdad, in 2005.
Seven members of Kilo Company, 3rd Battalion, 1st Marines were charged in connection with the incident. Charges against one have since been dropped in return for his testimony.
Many of the unit's personnel would have changed between the alleged Falluja incident and the Haditha case.
PHOTO CAPTION
Residents check for their missing relatives from bodies uncovered in a mass grave in Falluja, 50 km (30 miles) west of Baghdad, July 1, 2007. (REUTERS)
BBC