The chairman of the military's Joint Chiefs of Staff said Monday "it would be premature for me to judge" the outcome of a Pentagon investigation into the killing of as many as a dozen Iraqi civilians by Marines.

But at the same time, Marine Gen. Peter Pace said he believes its critically important to make the point that if certain service members are responsible for an atrocity there, "99.9 percent of our guys and gals are doing the job exactly the way they should be."

He also told FOX News that Gen. Mike Hagee, commandant of the Marine Corps, is going to reinforce the idea with the Marines "that that's what American fighting men and women do, they perform their mission with honor and dignity.

"And we'll let this investigation take its course and see where it goes and make it public," Pace said.

Pace's interview came a day after Rep. John Murtha, a decorated Marine war veteran and prominent critic of Iraq policy, said the incident could undermine U.S. efforts there more than the Abu Ghraib prison scandal did.

Murtha, D-Pa., also charged that the shootings last November at Haditha, a city in the Anbar province of western Iraq that has been plagued by insurgents, were covered up.

"Who covered it up, why did they cover it up, why did they wait so long?" Murtha said on Sunday "This Week" on ABC. "We don't know how far it goes. It goes right up the chain of command."

A bomb rocked a military convoy on Nov. 19, killing a Marine. Marines then shot and killed unarmed civilians in a taxi at the scene and went into two homes and shot other people, according to Murtha, who has been briefed by officials.

Iraqis who identified themselves as survivors of the killings described Marines shooting to death 19 people in three homes, among them a 77-year-old man in a wheelchair and a 4-year-old boy in one home and five children, ages 3 to 14, in another home, The New York Times reported Monday.

Those interviewed for the Times story said the men killed in the taxi were four students and the driver, all between the ages of 18 and 25. In one of the homes, according to the people interviewed for the Times story, Marines forced all the women to leave and then killed the four brothers they had detained.

Murtha said high-level reports he received indicated that no one fired upon the Marines or that there was any military action against the U.S. forces after the initial explosion. Yet the deaths were not seriously investigated until March because an early probe was stifled within days of the incident, he said.

"I will not excuse murder, and this is what happened," he said. "This investigation should have been over two or three weeks afterward and it should have been made public and people should have been held responsible for it."

Said Pace: "This investigation is ongoing. It would be premature for me to judge the outcome."

Asked how such a thing could have happened, he replied, "Fortunately, it does not happen very frequently, so there's no way to say historically why something like this might have happened. We'll find out."

Pace's predecessor, retired Gen. Richard Myers, told ABC's "Good Morning America" that he had "no idea" what happened but that "has been and there is an ongoing, thorough investigation."

Murtha repeated his view that the war in Iraq cannot be won militarily and needs political solutions, which he said were damaged by such incidents involving the U.S.

"This is the kind of war you have to win the hearts and minds of the people," he said. "And we're set back every time something like this happens. This is worse than Abu Ghraib."

The U.S. effort to win over Iraqis and others in the Arab world by fostering a democratic government was severely damaged when it was revealed that U.S. military personnel had abused and humiliated people held at Abu Ghraib, a prison outside of Baghdad.

The incident at Haditha has sparked two investigations -- one into the deadly encounter itself and another into whether it was the subject of a cover-up.

The second, noncriminal investigation is examining whether Marines sought to cover up what actually occurred that day and, in doing so, lied about having killed civilians without justification. The Marine Corps had initially attributed 15 civilian deaths to the car bombing and a firefight with insurgents, eight of whom the Marines reported had been killed.

A defense official, who spoke on condition of anonymity because he was not authorized to speak publicly, told The Associated Press on Friday that evidence gathered so far strongly indicated that the Haditha killings were unjustified.

Early this year, a videotape of the aftermath of the incident, showing the bodies of women and children, was obtained by Time magazine and Arab television stations. The military then undertook another investigation.

Sen. John Warner, the chairman of the Senate Armed Services Committee, said he would hold hearings on the killings but cautioned against reaching conclusions until the military concluded its investigation.

"There is this serious question, however, of what happened and when it happened and what was the immediate reaction of the senior officers in the Marine Corps when they began to gain knowledge of it," Warner said.

The Naval Criminal Investigative Service investigation into the shootings is not expected to be completed earlier than in June. Whether violations of the Uniform Code of Military Justice, including murder, would be pursued would be determined by a senior Marine commander in Iraq.

The NCIS also is conducting a criminal investigation into another incident, the death of an Iraqi civilian on April 26, involving Marines in Hamandiyah, west of Baghdad.