But investigators say unstable mental health was still likely to have been the "fundamental, underlying cause" of the attack at Fort Hood in Texas.
Specialist Ivan Lopez turned his gun on himself after killing three people and wounding 16 on Wednesday at the base, where more than a dozen people were killed by a soldier in 2009.
Lopez, an army truck driver from Puerto Rico, was undergoing treatment for depression and anxiety while being evaluated for post-traumatic stress disorder.
Lt Gen Mark Milley said there was a "strong indication" Lopez was involved in an argument shortly before the shooting, though it does not appear he targeted specific soldiers.
Mr Milley said: "We have very strong evidence he had a medical history that indicates unstable psychiatric or psychological condition. We believe that to be a fundamental, underlying cause."
The 34-year-old served four months in Iraq but saw no combat and he had previously demonstrated no apparent risk of violence.
Lopez bought the weapon he used in the attack at a store, called Guns Galore, near the base.