Ian David Long began his adult life by enlisting in the Marines and getting married. Years after a divorce and a discharge and months after shouting brought a mental health expert to his door, Long killed 12 people at a country music bar outside Los Angeles.

A former military machine gunner, the 28-year-old apparently killed himself after the Wednesday night attack in Thousand Oaks, the quiet suburb where he lived with his mother.

Neighbours said on Thursday they would hear loud, aggressive fights between the two. Authorities said they visited that ranch-style house only once, in April, after a neighbour reported yelling and crashing sounds.

Deputies found Long "was somewhat irate, acting a little irrationally" and called in a mental health specialist, Ventura County Sheriff Geoff Dean said. That specialist assessed Long but concluded he couldn't be involuntarily committed for psychiatric observation.

"The mental health experts out there cleared him that day," Mr Dean said on Thursday, though they were concerned he might be suffering from post-traumatic stress disorder because of his military service.