SAVANNAH -- Though they served in constant danger from enemy insurgents in Iraq, the deaths of Staff Sgt. Darris Dawson and Sgt. Wesley Durbin are tough to reconcile. Both men, the Army says, were fatally shot by a U.S. soldier in their own unit.

Few details of the slayings have been released. The soldiers were killed Sunday at a small patrol base in Tunis, Iraq, occupied by troops from Fort Stewart in Georgia. The alleged shooter was restrained by other soldiers and is being held in Iraq. He has not been charged and his name has not yet been released, Fort Stewart spokesman Kevin Larson said Thursday.

The case has left two grieving families with one unfathomable question: Why would an American soldier open fire and kill two comrades he'd been trained to protect with his own life?

"You should be able to trust your own soldiers," said Maxine Mathis, Dawson's stepmother. "I don't why he killed my son."

It's extremely rare that U.S. soldiers have slain their own in Iraq since the war began in 2003, especially when compared to the Vietnam War. Soldiers have been court-martialed or changed in four other Iraq cases:

— At Fort Lewis, Wash., Army Cpl. Timothy Ayers pleaded guilty in April to involuntary manslaughter in the fatal 2007 shooting of his platoon sergeant. Ayers said he thought the pistol he pointed at Sgt. 1st Class David A. Cooper Jr. was unloaded.

— Spc. Chris Rolan, an Army medic, pleaded guilty last year at Fort Benning, Ga., to unpremeditated murder in the 2005 slaying of Pvt. Dylan Paytas. Rolan shot Paytas four times with a handgun after a night of heavy drinking in Iraq.

— Staff Sgt. Alberto B. Martinez of the New York Army National Guard is in custody at Camp Lejeune, N.C., awaiting a court-martial for the murders of two superior officers. Prosecutors say he planted a mine that killed Capt. Phillip T. Esposito and 1st Lt. Louis E. Allen.

— Sgt. Hasan Akbar was sentenced to death at Fort Bragg, N.C., in 2005 for murder stemming from a grenade and rifle attack that killed two officers — Army Capt. Chris Seifert and Air Force Maj. Gregory Stone — in Kuwait early in the Iraq invasion of 2003.

"It's very, very unusual," said Dr. Clyde Flanagan, a former Army psychiatrist who now teaches the University of South Carolina Medical School. "Ordinarily in war, we look at the healing power of the unit a soldier is in. They take care of each other. Usually, they're willing to die for their guys."

Flanagan said soldiers in Iraq are under tremendous stress — from the threat of enemy fire and roadside bombs, from problems back home such as sick children or struggling spouses, even from the punishing desert heat.

And, like any workplace, there are conflicts among soldiers serving in the same unit.

Soldiers under tremendous stress are more likely to become agitated, less able to control their own anger, even become violent. In extreme cases, friends can even become foes, said Dr. Redford B. Williams, director of the Behavioral Medicine Research Center at Duke University.

"When you have a group of soldiers in Iraq, where you never know when the next car coming down the road is going to blow up on you, its not surprising some disagreements would escalate to the point where people are pulling guns," said Williams, a professor of psychology and psychiatry who has serve as a consultant to the U.S. Army War College.

It's still unclear what caused the shooting Sunday that killed Dawson, 24, of Pensacola, Fla., and Durbin, 26, of Dallas. Both men and their accused killer served in the 3rd Battalion, 7th Infantry Regiment based at Fort Stewart.

A defense official in Washington told The Associated Press the accused shooter is a sergeant who was in a meeting to discuss his leadership performance with Dawson, who was his squad leader, and Durbin, who was a fellow team leader in the squad. The official spoke on condition of anonymity because details have not been released.

"That young man had a family, too," Mathis, Dawson's stepmother, said of the alleged shooter. "And my heart goes out to them, too, because they love their son as much as we loved ours."

Durbin's parents declined to comment, according to a young man who answered the door Thursday at their Dallas home, where a large U.S. flag was posted on the front porch and cars and SUVs lined the circular driveway.

Incidents of American soldiers killing comrades and superior officers were much higher in Vietnam, especially as U.S. troops became more disillusioned with the war. Between 1969 and 1971, the Army reported 600 cases of fragging — so-named because attackers often used fragmentation grenades — that killed 82 Americans and injured 651.

"Considering the number of people who have rotated in and out of Iraq over the years, these incidents are amazingly rare," said John Pike, a defense analyst and director of GlobalSecurity.org. "With Vietnam you had roughly 10 times as many combat fatalities, but we have not had a 10th as many murders. So somebody must be doing something right."

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Bynum has covered the military based in Georgia since 2001