Sgt. John M. Russell was taken into custody outside the clinic following Monday's shooting at Camp Liberty, said Maj. Gen. David Perkins. It was the deadliest case of soldier-on-soldier violence since the Iraq war began in 2003.
Perkins gave few details of the shooting and said there were conflicting accounts of what happened.
He said Russell was referred to the clinic by his superiors, presumably because of concern over his mental state. Perkins said Russell was "probably" on his third tour of Iraq but was due to leave soon.
Perkins said the assailant's weapon had been taken away, but somehow he got a new gun, entered the clinic and opened fire.
In Washington, a Pentagon official said Russell was escorted to the clinic, but once inside argued with the staff and was asked to leave. After he and his escort drove away, Russell apparently seized the escort's weapon and returned to the clinic, said the official, speaking on condition of anonymity because the investigation was ongoing.