In addition to being charged in the deaths of the soldiers, U.S. Army Sgt. John Russell also was charged with one count of aggravated assault for opening fire on soldiers at a clinic Monday on the base near the Baghdad airport, CNN reported.
A motive hasn't been uncovered, officials said.
Two investigations have begun into the shootings, the military spokesman said. One is a criminal investigation in the crime. The second will examine what can be done to prevent such violent acts in the future and to determine, more broadly, whether the military is doing enough to address mental health concerns, the spokesman said.
The spokesman said it was believed Russell was nearing the end of his third tour in Iraq.
Russell's weapon was taken last week by his commander after concerns were expressed about the sergeant's mental state, based on Russell's comments and actions, CNN said. Officials said it was unclear how Russell got the second weapon and entered the clinic.
Investigators were sifting through multiple versions of Monday's events, military officials said.