As the community reels from a shooting rampage at Fort Hood, investigators are grappling for answers on why Spc. Ivan Lopez opened fire against fellow soldiers.
Investigators are looking at a possible altercation with a fellow soldier "that immediately preceded the shooting," said Lt. Gen. Mark Milley, the post's commanding general.
They are still piecing together the answer, but seem to be honing in on at least one thing that they say might have made the 34-year-old pull the trigger.
"We have very strong evidence that he had a medical history that indicates an unstable psychiatric or psychological condition. (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 Thursday.
Investigators say they haven't found any links to terrorism so far.
"At this point we have not yet ruled out anything whatsoever," Milley said. "We are committed to letting the investigation run its course."
The shooting blindsided victims, said Dr. Matthew Davis, medical director of trauma services at Scott and White Memorial Hospital.
"It was a shocking episode. It felt very surreal," he said after speaking to victims. "It was a very unexpected thing in the middle of a working day."
See updates on this story at CNN.com.