For some local veterans, the weekend incident in which a U.S. Marine was videotaped opening fire on a wounded Iraqi was a reminder of how uncertain and chaotic war can be.

On Saturday, a Marine killed an Iraqi in a Fallujah mosque after shouting that the injured man was "faking he's dead." The incident was recorded by an embedded NBC correspondent and aired on Monday in an abbreviated version in the U.S. and in its entirety throughout the Arab world.

Yesterday, the 1st Marine Division said it started an investigation to determine whether the soldier, whose name has not been released, was acting in self-defense or violated military law by killing injured combatants who are out of action. But already, the incident poses problems for Prime Minister Ayad Allawi and his U.S. backers at a time when Iraqi authorities are seeking to contain a backlash among Sunnis.

Without passing judgment, local World War II and Vietnam veterans said they understood how something like this could have happened.

"It does show you what human beings can do to other human beings," said Don Murray, an 80-year-old writer and Boston Globe columnist from Durham, who served on the front lines in Europe during World War II. "It goes on in all wars. It comes out of some funny combination of fear and power."

Veterans said the few minutes of footage didn't give enough context to evaluate the situation. They didn't know, for example, what the unit had experienced before entering the mosque and whether soldiers had been subjected to unusual amounts of stress.

"It was a mental lapse that occurred,"said Ken Leidner, the director of the State House visitors' center, who directed air strikes as a radio operator in Vietnam. "Should there have been steps two and three before this guy got shot? Probably, but we weren't there. We don't know what was going on, what kind of pressure these guys were under before they stepped in."

The day before the incident, another member of the Marine's squad had been killed when he touched the dead body of an insurgent who was booby-trapped, the NBC correspondent, Kevin Sites, said. The Marine himself had been injured in the face.

The five wounded Iraqis had been left in the mosque by a different Marine squad who had battled them on Friday and killed 10 others, Sites said. It is unclear whether the five who were left behind were prisoners, but Sites said three more of them were also killed by the Marines.

Amnesty International and Human Rights Watch echoed calls by the top U.N. human rights official, Louise Arbour, for an investigation of possible abuses in Fallujah.

"The deliberate shooting of unarmed and wounded fighters who pose no immediate threat is a war crime under international law, and there is therefore an obligation on the U.S. authorities to investigate all such reports and to hold perpetrators of such crimes accountable before the law,"Amnesty said in a statement.

All of the veterans interviewed for this story said they thought this incident was an isolated one, caused by the particulars of the situation and not a widespread problem of criminal violence by U.S. troops.

"There are moments when something just clicks because of the kind of pressures you've been under," said Mark Smith of Newbury, a rifleman in Vietnam. "And then five minutes later, you might do quite the opposite, given the same situation."

Jean Stimmell, a Vietnam veteran and therapist from Northwood who has counseled veterans suffering from post-traumatic stress disorder, said the Marine wasn't living with the logic of the civilian world, but in a kill-or-be-killed environment.

"You're not really in a rational state, you're in the middle of an adrenaline flight-or-fight reaction, which is probably what this young soldier was going through, coupled with the fact that he was (injured) the day before," Stimmell said. "In his mind, there was a split second or he'd be dead. That was his psychological reality."

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