Army Sgt. John M. Russell is suspected of being the gunman who shot one doctor from both the Navy and Army, and three enlisted Army soldiers who were seeking treatment at the clinic, according to the statement.
"The suspect ... Sgt John Russell is charged with five specifications of murder and one specification of aggravated assault," the statement said, adding that he was currently in military police custody.
Monday's incident was the deadliest soldier-against-soldier attack since the beginning of the Iraq war.
Having already served multiple tours in Iraq, Russell was due to leave Iraq soon.
"This suspected individual was apprehended outside of the clinic shortly after the shots were heard," Major-General David Perkins, the spokesman for U.S. forces in Iraq said, adding that the investigation will "examine the steps taken to see if we can reduce the possibility of another event like it occurring in the future."
Many details about the attack have not been released due to the ongoing investigation.
Russell, who served with the 54th Engineering Battalion based in Bamberg, Germany, had been referred to the clinic at Camp Liberty near the Baghdad airport as a patient, according to Maj. Gen. Daniel Bolger.
His commander had earlier determined that it was best he have his weapon taken away, Perkins said, but Russell had gotten another one which he allegedly used in his assault. Off-duty soldiers are not supposed to have loaded weapons on American military bases in Iraq.
"Suffice to say by his actions or what he had said his chain of command was concerned enough that 1. he was in a formalized counseling process and going for mental health assistance and number 2. there was concern he should not have a weapon, so again the chain of command confiscated his weapon which is something that is not done routinely to average soldiers," Perkins told reporters, according to the Christian Science Monitor.
At the Pentagon, Joint Chiefs of Staff chief Admiral Mike Mullen expressed concern about sufficient mental health care for servicemen and women, as well as sufficient time at home between deployments. "It does speak to me about the need for us to redouble our efforts in terms of dealing with the stress," Mullen said.
President Barack Obama said he was "shocked and deeply saddened" by the incident.
The names of the victims have not been released, awaiting notification of the families.
The U.S. military is coping with a growing number of stress cases among soldiers in Iraq and Afghanistan -- many of whom are on their third or fourth combat tours. Some studies suggest that about 15 percent of soldiers returning from Iraq suffer from some sort of emotional problems.
