The Marine Corps initially reported 15 deaths and said they were caused by a roadside bomb and an ensuing firefight with insurgents. A separate investigation is seeking to determine if Marines lied to cover up the killings.
The official, who discussed the matter on condition of anonymity because he was not authorized to discuss the yet-to-be-completed investigation, said the evidence developed by investigators strongly indicates the killings last November in the insurgent-plagued city of Haditha in the western province of Anbar were unjustified.
The official did not disclose specific evidence. The incident, if confirmed, could be the most serious case of criminal misconduct by U.S. troops during three years of combat in Iraq.
Members of Congress have been told eight marines and one Navy corpsman were present when the killings took place, but a much larger number knew what had happened because another unit went in to take pictures of the bodies after they had been shot, reports CBS News correspondent David Martin.
One photo shows a woman and child in a kneeling position, shot as if they were praying or begging for mercy. Another photo shows a woman and children shot as they lay in bed. Many of the Iraqis were shot in the head or chest at close range, Martin reports.
In an indication of how seriously the Marines consider it, their top officer, Gen. Michael W. Hagee, on Thursday to reinforce the need to adhere to Marine values and standards of behavior and to avoid the use of excess force.
"We do not employ force just for the sake of employing force. We use lethal force only when justified, proportional and, most importantly, lawful," Gen. Michael W. Hagee, the Marine Corps commandant, wrote in a statement issued by his office. Aides said it was the basis of remarks he intended to make to Marines in Iraq this week.
A spokesman at Marine Corps headquarters in the Pentagon, Lt. Col. Scott Fazekas, declined to comment on the status of the investigation. He said no information would be provided until the probe was completed.
However, a lawyer for one of the Marines told Martin that there is a strong possibility some of them will be charged with murder.
According to a congressional aide, lawmakers were told in a briefing Thursday that it appears as many as two dozen civilians were killed in the episode at Haditha. And they were told that the investigation will find that "it will be clear that this was not the result of an accident or a normal combat situation."
Another congressional official said lawmakers were told it would be about 30 days before a report would be issued by the investigating agency, the Naval Criminal Investigative Service.
Both the House and Senate armed services committees plan to hold hearings on the matter.
Hagee met with top lawmakers from those panels this week to bring them up to date on the investigation.
"I can say that there are established facts that incidents of a very serious nature did take place," Sen. John Warner, chairman of the Senate panel, said Thursday. He would not provide details or confirm reports that about 24 civilians were killed. He told reporters he had "no basis to believe" the military engaged in a cover-up.
In the Haditha case, videotape aired by an Arab television station showed images purportedly taken in the aftermath of the encounter: a bloody bedroom floor, bullet holes in walls and bodies of women and children. An Iraqi human rights group called for an investigation of what it described as another deadly mistake that had harmed civilians.
On May 17, Rep. John Murtha, D-Pa., a decorated former Marine, said Marine Corps officials told him the toll in the Haditha attack was far worse than originally reported and that U.S. troops killed innocent women and children "in cold blood." He said that nearly twice as many people were killed than first reported, maintaining that U.S. forces are "overstretched and overstressed" by the war in Iraq.
Pentagon spokesman Eric Ruff said Friday he believes the investigation is winding down, but he would not comment on what the evidence shows.
Ruff would not characterize the extent of Defense Secretary Donald H. Rumsfeld's concern about the matter, but said he is being kept apprised of the investigations.
Ruff said he did not expect any announcements in the next few days.