A US soldier opened fire on fellow troops at one of the main US bases in Baghdad today, killing five, the US military said.
"The shooter is a US soldier and he is in custody," said Marine Corps Lieutenant Tom Garnett, a US military spokesman in Iraq.
US officials said they would charge him later in the day.
The shooting took place around 2pm local time at Camp Liberty, a sprawling, dusty base located next to the Baghdad airport.
"This is certainly an unexpected and tragic event," said Pentagon spokesman Bryan Whitman.
"Any time we lose one of our own, it affects us all," said Colonel John Robinson, a US spokesman in Baghdad.
It was not the first incident of violence by US soldiers against their fellow troops in the course of the war in Iraq.
In perhaps the most well-known case, two officers were killed and 14 soldiers were wounded when a US Army sergeant who had converted to Islam, Hasan Akbar, launched a grenade attack at a base in Kuwait just before the 2003 invasion.
Attacks like the latest one raise questions about the toll that six years of continuous warfare have taken on the US military and individual soldiers, many of whom have seen multiple tours.
According to the US Department of Defense, nearly 20 percent of veterans of the wars in Iraq and Afghanistan report symptoms of Post-Traumatic Stress Disorder (PTSD).
The number of US soldiers killed in today's shooting matched the death toll from a truck bomb in northern Iraq last month, which was the single deadliest incident for US troops in more than a year.
No further details about the shooting were immediately available.
Reuters