A U.S. soldier serving in Iraq was in custody Monday night. He's accused of killing five of his fellow soldiers at Camp Liberty, an American base near Baghdad International Airport.
The shooter opened fire inside a clinic where troops go for help for combat stress and personal issues.
Officials aren't sure what triggered the attack.
"The President's heart goes out to the families and friends of all the service members involved in this horrible tragedy," White House Spokesman Robert Gibbs said.
The shooting comes as the military continues to grapple with over stressed troops and high suicide rates.
The army, for its part, has created stress control teams and makes soldiers undergo mandatory suicide prevention training -- a program chaplains are involved in but it's a massive issue to tackle.
Admiral Mike Mullen, Chairman of the Joint Chiefs of Staff, recently told reporters the U.S. is already on pace this year to exceed the number of suicides committed by members of the military last year.
"It does speak to me about the need for us to redouble our efforts in terms of dealing with the stress, dealing with the issue of those kinds of things, and it also speaks to the issue of multiple deployments, increasing dwell time," Mullen said. "All those things that we're focused on to try and relieve that stress."
President Obama says he's working with military leaders to do all he can to keep American men and women serving in the military safe.