A U.S. soldier opened fire on fellow troops at a counseling center on a military base Monday, killing five before being taken into custody, the U.S. command and Pentagon officials said.

The soldier who shot is in custody and would be charged later today.

The cause of the shooting remains unclear however this incident draws attention to the issue of combat stress and morale after six years of war as the mission of the 130,000-strong force transforms to one of training and mentoring the Iraqis.

It was not the first incident of violence by U.S. soldiers against their fellow troops in the course of the war in Iraq. Attacks like the latest one raise questions about the toll that six years of continuous warfare have taken on the U.S. military and individual soldiers.

Attacks on fellow soldiers, known as fraggings, were not uncommon during the Vietnam war but are believed to be rare in Iraq and Afghanistan.