In the search for answers after a shooting that left 12 people dead at the Borderline Bar & Grill in Thousand Oaks, Calif., much attention has been focused on the gunman’s time in the Marine Corps, wondering if post-traumatic stress disorder from combat in Afghanistan left him primed to kill.

Dominique Colell says that’s a mistake.

She was a track coach at Newbury Park High School when Ian D. Long, whom authorities identified as the gunman, was a senior on the team. Even then, years before he deployed to Afghanistan, she said, Mr. Long was defiant and angry and eventually assaulted her on the track.

“This shouldn’t be blamed on PTSD. He was disturbed long before that,” she said Friday. “The military may have provided some training, but he had a propensity to violence before he ever joined.”

While law enforcement authorities and some who knew him have speculated that Mr. Long, 28, who deployed to Afghanistan in 2010, may have suffered from PTSD, no evidence has been made public that it was ever diagnosed or that he was treated. President Trump joined the chorus on Friday, calling the gunman a “sick puppy.”