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 aimed at determining if Marines lied to cover up the events, which included the deaths of women and children.

If confirmed as unjustified killings, the episode could be the most serious case of criminal misconduct by U.S. troops during three years of combat in Iraq. Until now the most infamous occurrence was the Abu Ghraib prisoner abuse involving Army soldiers, which came to light in April 2004 and which President Bush said Thursday he considered to be the worst U.S. mistake of the entire war.

The defense official discussed the matter Friday only on condition of anonymity because he was not authorized to talk publicly about the investigation. He said the evidence found thus far strongly indicated the killings in the insurgent-plagued city of Haditha in the western province of Anbar were unjustified. He cautioned that the probe was not finished.

Once the investigation is completed, perhaps in June, it will be up to a senior Marine commander in Iraq to decide whether to press charges of murder or other violations of the Uniform Code of Military Justice.

Three officers from the unit involved — 3rd Battalion, 1st Marine Regiment, 1st Marine Division, based at Camp Pendleton, Calif. — have been relieved of duty, although officials have not explicitly linked them to the criminal investigation.

In an indication of how concerned the Marines are about the implications of the Haditha case, their top officer, Gen. Michael W. Hagee, flew to Iraq on Thursday. He was to reinforce what the military said was a need to adhere to Marine values and standards of behavior and to avoid the use of excess force.

"Many of our Marines have been involved in life or death combat or have witnessed the loss of their fellow Marines, and the effects of these events can be numbing," Hagee said a statement announcing his trip. "There is the risk of becoming indifferent to the loss of a human life, as well as bringing dishonor upon ourselves."

A spokesman at Marine Corps headquarters in the
Pentagon, Lt. Col. Scott Fazekas, declined to comment on the status of the Haditha investigation. He said no information would be provided until the probe was completed.

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.

The New York Times reported on Friday that the civilians killed at Haditha included five men who had been traveling in a taxi and others in two nearby houses. The newspaper quoted an unidentified official as saying it was a sustained operation over as long as five hours.

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 (news, bio, voting record), 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.

Separately, the Marines announced this week that a criminal investigation was under way in connection with an alleged killing on April 26 of an Iraqi civilian by Marines in Hamandiyah, west of Baghdad. No details about that case have been made public.

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, walls with bullet holes and bodies of women and children. An Iraqi human rights group called for an investigation of what it described as a deadly mistake that had harmed civilians.

On May 17, Rep. John Murtha (news, bio, voting record), D-Pa., a former Marine, said 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 as first reported and maintained that U.S. forces were "overstretched and overstressed" by the war in Iraq.

Pentagon spokesman Eric Ruff said Defense Secretary Donald H. Rumsfeld was being kept apprised. Ruff said he did not expect any announcements in the next few days.