Investigators say unstable mental health may be the underlying cause of a soldier's fatal shooting rampage at Fort Hood this week. But they also believe an argument with another service member likely preceded the attack.

Spc. Ivan Lopez turned his gun on himself after killing three people and wounding 16 others Wednesday at the nation's largest Army post. It's the same Texas military base where more than a dozen people were fatally shot by a soldier in 2009.

Lt. Gen. Mark Milley is Fort Hood's senior officer. He says Lopez's psychological issues may be the "fundamental, underlying cause" of the attack.

Lopez had been undergoing treatment for depression and anxiety while being evaluated for post-traumatic stress disorder.

Milley also says Lopez was likely involved in an argument before the shooting.