Murder charges may be brought against some Marines for what may be the worst atrocity committed by U.S. military personnel in Iraq, a senior Pentagon official said Friday.

The official said Marines were likely responsible for killing as many as two dozen unarmed civilians, including women and children, in Haditha last November. A separate investigation is under way into whether Marines tried to cover up the killings. The official requested anonymity, citing the ongoing criminal investigation of the incident.

The Marines initially reported that one Marine and 15 Iraqi civilians had been killed in crossfire when U.S. forces responded to an insurgent attack on Nov. 19. The first report to the contrary surfaced in March, when Time magazine quoted witnesses saying the Marines "went on a rampage after the attack, killing 15 unarmed Iraqis in their homes, including seven women and three children."

In addition, photos taken of the scene reportedly do not support the Marines' original account of how the incident evolved.

A Pentagon official said that the Time report was "fairly benign" compared to what the investigation by the Naval Criminal Investigative Service and Multi-National Forces Iraq has uncovered. Lawmakers have been told that the results of the probe will be issued in about 30 days, a congressional official said.

Three Marines have been relieved of their command, though the reasons have not been announced. Any Marines found guilty of murder, a capital offense, could face the death penalty.

Last week, Rep. John Murtha, a decorated Vietnam veteran and former Marine who is the ranking Democrat on the House Defense Subcommittee, told reporters after being briefed on the probe that the killings were committed "in cold blood." On Thursday, Senate Armed Services Committee Chairman John Warner told reporters that "incidents of a very serious nature did take place."

A Pentagon spokesman said Friday that he believes the military's investigation is winding down, but that no announcements should be expected in the next few days.

Eric Ruff, the Pentagon press secretary, did not comment on reports that murder charges were possible' he said Defense Secretary Donald H. Rumsfeld was being kept apprised of the investigations.

"Needless to say, we have to take seriously allegations of that type, and they're under investigation and they will then be handled in the normal order of things," Rumsfeld said on FOX News last week.

On Wednesday, the Marines announced that a separate investigation had been launched into the alleged killing of an Iraqi civilian in Hamandiyah by Marines in April. As many as eight Marines who may have been involved were relieved of their command and ordered back to the United States, an official said.

A Pentagon official said the death occurred "outside of any authorized operations" and that some Marines have already confessed to involvement.

On Thursday, Marine Corps Commandant Gen. Michael W. Hagee flew to Iraq to warn his troops against allowing the unrelenting insurgent violence to leave them callous to human suffering and the loss of life.

"We do not employ force just for the sake of employing force. We use lethal force only when justified, proportional and, most importantly, lawful," Hagee 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.

"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 in his statement. "There is the risk of becoming indifferent to the loss of a human life, as well as bringing dishonor upon ourselves."

"To a Marine, honor is more than just honesty; it means having uncompromising personal integrity and being accountable for all actions," Hagee said. He urged all Marines to have the moral courage to "do the 'right thing' in the face of danger or pressure from other Marines."

He referred to "recent serious allegations about actions of Marines in combat," but he did not specifically cite the two cases — one from last November and the other in April — of alleged killings of civilians.

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 and discussed the November and April incidents.

FOX News' Mike Emanuel, Nick Simeone and The Associated Press contributed to this report.