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U.S. Defense Secretary Donald H. Rumsfeld on Friday defended the training and conduct of U.S. troops and said incidents like an alleged massacre of Iraqi civilians should not happen.

"We know that 99.9 percent of our forces conduct themselves in an exemplary manner. We also know that in conflicts things that shouldn't happen, do happen," he said.

While he said he cannot comment on the investigation because it could taint the case, he said, "We don't expect U.S. soldiers to act that way, and they're trained not to."

Rumsfeld spoke to reporters at a defense ministers conference in Singapore.

Lawyer Says Seven Marines, One Sailor Could Face Murder Charges for Iraqi Slaying

The military is conducting two investigations into the alleged murder of about two dozen Iraqi citizens by U.S. Marines in Haditha, an insurgent stronghold in western Iraq. One is a criminal probe into the actual events that day last November, and the other is looking into whether soldiers told the truth about what happened or if there was a cover-up.

In Washington, a senior U.S. military officer said in a video teleconference with reporters at the Pentagon that commanders realize that troops involved in counterinsurgency operations "could snap" in the face of severe pressures.

"When you're in a combat theater dealing with enemy combatants who don't abide by the law of war, who do acts of indecency, soldiers become stressed, they become fearful," said Brig. Gen. Donald Campbell, the chief of staff at the top U.S. military headquarters in Baghdad. "It's very difficult to determine in some cases on this battlefield who is a combatant and who is a civilian.

"It doesn't excuse the acts that have occurred, and we're going to look into them," he added. "But I would say it's stress, fear, isolation and, in some cases, they're just upset. They see their buddies getting blown up on occasion, and they could snap."

In an interview with WOAI-AM, a radio station in San Antonio, Texas, Attorney General Alberto Gonzales said the allegations against U.S. troops, if true, are "very, very serious" and that the world will see a thorough military investigation.

"If people are found to have committed crimes, those people will be held responsible and they will be held accountable," Gonzales said. "The President expects that, and I know the leadership in the military wants to see that happen as well."

He added that he thinks "people will be comfortable that the United States has dealt with people, if the facts are true, that the United States has dealt with people in a way that they're held to account for not meeting the standards that we expect of all of our men and women in uniform."

Rumsfeld said he has not spoken to Iraqi Prime Minister Nouri al-Maliki about the matter. Al-Maliki Thursday upbraided the U.S. military over allegations that Marines killed the unarmed civilians in Haditha, calling the killings "a horrible crime" in his strongest public comments on the subject since his government was sworn in last month.

Rumsfeld said U.S. soldiers receive training in the laws of war and the Geneva Conventions. But he also said that Gen. George Casey, the top U.S. military commander in Iraq, has taken appropriate steps to make sure U.S. troops "are fully aware of the high standards to which they are being held."

The U.S. military on Thursday ordered coalition troops in Iraq to undergo special training in ethics and "the values that separate us from our enemies" in the wake of the Haditha allegations.