A criminal probe by the Naval Criminal Investigative Service, which handles criminal inquiries involving marines, has not been completed and no final decisions on charges have been made, said the official on Friday, speaking on condition of anonymity because of the sensitivity of the matter.
The Los Angeles Times reported that investigators were expected to call for charges including murder, negligent homicide, dereliction of duty and filing a false report.
The investigation involves a November 19 incident in Haditha, about 220km northwest of Baghdad.
The military has said 15 civilians were killed, while a senior Republican legislator last week put the number at about 24.
The Los Angeles Times reported that military investigators had concluded that a dozen marines acted improperly in an incident in which US soldiers, after a marine was killed by a roadside bomb, wantonly killed unarmed civilians, including women and children, and then tried to cover up the incident.
Pentagon press secretary Eric Ruff declined to comment on the findings or possible charges.
Ruff said he believed investigators were "towards the end" of their probe, but added: "I don't think there's anything that's imminent."
The defence official noted that criminal investigations into deaths could lead to murder charges, but was not more specific about possible charges.
The civilian deaths came after Marine Lance Corporal Miguel Terrazas, of El Paso, Texas, was killed by a roadside bomb.
There are 21,000 US marines
serving in Iraq
The military initially said the blast also killed 15 civilians.
A video of people killed in the incident, given to Reuters in March by Iraq's Hammurabi Organisation for Monitoring Human Rights and Democracy, showed corpses lined up at the local morgue with bullet wounds in the head and chest.
The video showed houses with bullet holes in the walls, pieces of human flesh, pools of blood, and clothes and pots scattered on floors. Residents described a rampage by marines.
The marines involved were with the 3rd Battalion, 1st Marine Regiment of the 1st Marine Division, based at Camp Pendleton, California.
Relieved of duty
The military said in April the battalion commander and two company commanders had been relieved of duty.
Lieutenant-Colonel Scott Fazekas, a Marine Corps spokesman at the Pentagon, said: "The investigations are ongoing, therefore any comment at this time would be inappropriate and could undermine the investigatory and possible legal process."
Marine Corps leaders have briefed US legislators in recent days on the Haditha investigation and another into the role of several US soldiers in the death of one civilian last month west of Baghdad.
Women and children were among
those believed killed by marines
Legislators have emphasised the seriousness of the incidents.
Last week, Representative John Murtha, a Pennsylvania Democrat and retired marine, said of Haditha: "Our troops overreacted because of the pressure on them, and they killed innocent civilians in cold blood. And that's what the report is going to tell."
There are 21,000 marines serving in Iraq in one of the most violent regions of the country; more than 700 have died since the war began in 2003.
General Michael Hagee, the Marine Corps commandant, flew to Iraq on Thursday for a series of meetings with marines to emphasise the need to follow the laws of war, the Geneva Conventions and rules of engagement set by the military.