A former high school coach of the gunman who killed 12 people at a Southern California bar says he was a “sadistic” teenager who joined the military so he would “kill for his country”.

Ian David Long opened fire with his handgun after storming the Borderline Bar & Grill In Thousand Oaks. He first shot a group of security guards and employees near the entrance before aiming at victims on the dance floor.

Evie Cluke says that years before his shooting rampage, the 28-year-old former marine was already displaying volatile and intimidating behaviour.

The educator, who coached Long on Newbury Park High School's track team in 2007 and 2008, said he was a “ticking time bomb” who constantly lost his temper, threw tantrums and would scream at coaches when he didn't like their decisions.

Recalling her experience of working with him, she said that despite repeated complaints to school administrators about his behaviour, he was never disciplined.

“Just sadistic. ... He was out of control,” Ms Cluke said. “He would scream and cuss and his face would turn bright red and people would actually back away from him.”

She said she also witnessed him assault fellow coach Dominique Colell by grabbing her bottom and her midsection after she confiscated a mobile phone.

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On another occasion he used his hand to mimic shooting Ms Colell. “When Dominique turned around and saw that, she turned pale as a ghost and it was very, very scary,” Ms Cluke added.

Long was kicked off the track team immediately after he assaulted Ms Colell, but her decision was reversed by the boy's track coach who said that it could compromise his goal of joining the Marines.

Long, who would go on to serve in Afghanistan, rejoined the team after apologising in front of several coaches and administrators.

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“He said he wanted to be in the Marines because he wanted to go fight in the war for our country and he wanted to kill for our country,” she said.

“When you hear somebody say they want to be in the military because they want to kill people in the name of our country, that's chilling. It chilled me right down to my bones.”

While investigators have not commented on whether mental illness played a role in the Long’s attack, a mental health specialist who assessed him in April after officers responded to a call about his agitated behaviour, suspected that he might be suffering from post-traumatic stress disorder (PTSD).

Although a mental health crisis team was called to the scene, a specialist concluded there were no grounds to have him involuntarily committed.