FILE - In this Thursday, Nov. 5, 2009, file photo, an entrance is shown to Fort Hood Army Base in Fort Hood, Texas. Fort Hood says there’s been a
FILE - In this Thursday, Nov. 5, 2009, file photo, an entrance is shown to Fort Hood Army Base in Fort Hood, Texas. Fort Hood says there's been a shooting at the Texas Army base and that there have been injuries, on Wednesday, April 2, 2014. (AP Photo/Jack Plunkett)

FORT HOOD, Texas — Unstable mental health may be the "fundamental, underlying cause" of a soldier's shooting rampage at Fort Hood that left four people dead, though an argument with another service member likely preceded the attack, according to investigators.

Spc. Ivan Lopez turned his gun on himself after killing three people and wounding 16 others Wednesday at the sprawling Texas military base, where more than a dozen people were fatally shot by a soldier in 2009. An Army truck driver from Puerto Rico, Lopez was undergoing treatment for depression and anxiety while being evaluated for post-traumatic stress disorder, base officials said.

Lt. Gen. Mark Milley, senior officer at the nation's largest Army base, said there was a "strong indication" that Lopez was involved in a verbal altercation shortly before the shooting, though it doesn't appear he targeted specific soldiers during the attack. But investigators are also focusing on his mental health.

"We have very strong evidence that he had a medical history that indicates unstable psychiatric or psychological condition," Milley said. "We believe that to be a fundamental, underlying cause."

Lopez had reported to medical personnel that he'd suffered a traumatic brain injury. The 34-year-old served four months in Iraq but saw no combat, and he had previously demonstrated no apparent risk of violence.