US

Argument likely preceded Fort Hood shooting rampage by gunman with 'unstable' mental condition

  • This undated photo provided by Glidden Lopez shows Army Spc. Ivan Lopez. Authorities said Lopez killed three people and wounded 16 others in a shooting at Fort Hood, Texas, on Wednesday, April 2, 2014, before killing himself. Investigators believe his unstable mental health contributed to the rampage. (AP Photo/Courtesy of Glidden Lopez)

    This undated photo provided by Glidden Lopez shows Army Spc. Ivan Lopez. Authorities said Lopez killed three people and wounded 16 others in a shooting at Fort Hood, Texas, on Wednesday, April 2, 2014, before killing himself. Investigators believe his unstable mental health contributed to the rampage. (AP Photo/Courtesy of Glidden Lopez)  (The Associated Press)

  • Lt. Gen. Mark Milley, left,and U.S. Sen. John Cornyn, right, talk to the media near Fort Hood's main gate, Thursday, April 3, 2014, in Fort Hood, Texas. A soldier opened fire Wednesday on fellow service members at the Fort Hood military base, killing three people and wounding 16 before committing suicide. (AP Photo/Eric Gay)

    Lt. Gen. Mark Milley, left,and U.S. Sen. John Cornyn, right, talk to the media near Fort Hood's main gate, Thursday, April 3, 2014, in Fort Hood, Texas. A soldier opened fire Wednesday on fellow service members at the Fort Hood military base, killing three people and wounding 16 before committing suicide. (AP Photo/Eric Gay)  (The Associated Press)

  • This undated photo provided by Glidden Lopez shows Army Spc. Ivan Lopez. Lopez killed three people and wounded 16 others in a shooting at Fort Hood, Texas, on Wednesday, April 2, 2014, before killing himself. Investigators believe his unstable mental health contributed to the rampage. (AP Photo/Courtesy of Glidden Lopez)

    This undated photo provided by Glidden Lopez shows Army Spc. Ivan Lopez. Lopez killed three people and wounded 16 others in a shooting at Fort Hood, Texas, on Wednesday, April 2, 2014, before killing himself. Investigators believe his unstable mental health contributed to the rampage. (AP Photo/Courtesy of Glidden Lopez)  (The Associated Press)

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.