Fort Hood gunman Ivan Lopez may have had a verbal altercation with a fellow soldier before he opened fire, killing three people and wounding 16 others, before turning the gun on himself at the army base on Wednesday, but otherwise showed no recent signs of violence, authorities said Thursday. 

Investigators are looking into the possibility that Lopez had words with a solder in an incident “that immediately preceded the shooting,” said Lt. Gen. Mark Milley, the post’s commanding general.

“We’re trying to figure out what the trigger event was,” Milley said.

Investigators are working to determine whether the shooting was pre-meditated, Milley told reporters.

While the trigger for the shootings is still unclear, investigators believe Lopez’ unstable mental health history is a key factor.

“We have very strong evidence that he had a medical history that indicates an unstable psychiatric or psychological condition,” Milley said Thursday.

“(We’re) going through all records to ensure that is, in fact, correct. But we believe that to be the fundamental underlying causal factor,” Milley said.

Though nothing is being ruled out by investigators at this time, there is no evidence that the shooting was linked to terrorism, national or international, Milley explained.

Army Secretary John McHugh said the soldier saw no combat during a four-month deployment to Iraq as a truck driver from August to December 2011.

Lopez saw a psychiatrist in March and showed no “sign of any likely violence either to himself or others,” McHugh said.

His record shows “no involvement with extremist organizations of any kind,” McHugh added.

Lopez was armed with a .45 caliber Smith & Wesson and turned the gun on himself when confronted by a female military police officer in a parking lot of the base, near Killeen, Texas, on Wednesday. Lopez, who served four months in Iraq in 2011, was married with four children and had arrived at Fort Hood in February.

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