An American Army sergeant shot and killed five fellow soldiers following an altercation at a military counseling centre in Iraq yesterday.
The attack drew attention to the issues of combat stress and morale among soldiers serving multiple combat tours over six years of war.
The suspect had been disarmed after an incident at the center but returned with another weapon, according to a senior military official in Washington, who spoke on condition of anonymity because the investigation into the shootings was ongoing.
Attacks on fellow soldiers, known as fraggings, were not uncommon during the Vietnam war but are believed to be rare in Iraq and Afghanistan.
A brief US military statement said the assailant was taken into custody following the shooting at Camp Liberty, a sprawling US base on the western edge of Baghdad near the city's international airport.
President Barack Obama, who visited an adjacent base last month, said in a statement that he was "shocked and deeply saddened" by the report, adding that "my heart goes out to the families and friends" of all those involved "in this horrible tragedy."
After a meeting with Defence Secretary Robert Gates, Obama said he would make sure "that we fully understand what led to this tragedy" and will do everything possible "to ensure that our men and women in uniform are protected as they serve our country so capably and courageously in harm's way."
The military statement in Baghdad said nobody else was hurt, but military officials in Washington said one person was wounded. The names of the victims and suspect were not released.
A military official said the sergeant had been involved in a verbal altercation at the centre. His service weapon was taken from him for his own protection and he was driven back to the centre later in the day.
The official said that when the sergeant returned he had another weapon. It was unclear whether he was returning under orders or of his own volition.
Another senior military official said the suspect was a patient at the clinic. The official did not know what relationship the shooter had to those he killed. ABC News, without identifying a source, reported that two of the dead were clinic staff and three were soldiers waiting for treatment.
The network also aired video of the stress clinic it said was taken just days ago. It showed the clinic's commander, Lt. Col Beth Salisbury, giving a tour of the clinic and describing certain areas where people cannot carry weapons. Salisbury is from the US Army Reserve 55th Medical Company based in Indianapolis.
At the Pentagon, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said the shooting occurred "in a place where individuals were seeking help."
"It does speak to me about the need for us to redouble our efforts in terms of dealing with the stress," Mullen said.
The US military is coping with a growing number of stress cases among soldiers in Iraq and Afghanistan - many of whom are on their third or fourth combat tours. Some studies suggest that about 15 per cent of soldiers returning from Iraq suffer from some sort of emotional problems.
AP