The U.S. military says an American soldier has killed five fellow soldiers at a U.S. base in Baghdad.
Military officials say the alleged attacker is in custody.
U.S. media report the killings took place Monday at a stress counseling clinic at Camp Liberty, on the outskirts of the capital.
The White House says President Barack Obama is "shocked" by the incident. Secretary of Defense Robert Gates expressed his horror and regret over the shooting. Gates said the shooting will receive high priority attention from the military.
Also Monday, Iraqi police say a senior officer was assassinated in a drive-by shooting in Baghdad.
Officials say unknown gunmen shot Abdul Hussein Mohsen al-Kadhemi as he drove through the center of the capital.
This is the second attack on a high-ranking police officer in recent days. On Saturday, an Iraqi police general was fatally shot near the southern city of Basra.
Iraq has seen a recent upsurge in violence. April was one of the deadliest months in the last year, with a reported 355 Iraqis killed.
Some information for this report was provided by AFP and AP.