Former machine gunner Ian David Long – who reportedly suffered from PTSD – used an illegally-modified handgun to target drinkers at the Borderline Bar & Grill in Thousand Oaks.
The former Marine donned a black trenchcoat and ski mask before storming the bar on a student night to unleash around 30 bullets in a two and half minute killing spree.
Now the lone gunman's neighbours have revealed the shooter's own mother “lived in fear” the former soldier would one day go off the rails.
Richard Berge told CNN said she told him she was hugely concerned about her son adding “she was… kind of beside herself, she didn’t know what to do because he wouldn’t get help.”
Divorced Long – who turned his gun on himself after his murder spree – lived with his mother, Colleen Long, and three German shepherds in a quiet suburban home in Newbury Park.
Earlier this year, police were reportedly summoned to the home after a bullet had been fired through one of the home's walls.
The street was blocked off, and police spent hours trying to get Long to peacefully come outside, reports the Wall Street Journal.
“They couldn’t get him out for a long time, like half the day,” said Mr Berge.
Other neighbours said Long was a veteran suffering from Post Traumatic Stress Syndrome (PTSD).
One said: "I have no idea what he was doing with a gun".
The Ventura County sheriff confirmed in a briefing Thursday that deputies responded to a disturbance at the Longs’ home in April and mental-health specialists were dispatched.
However, they determined that Long wasn’t a threat and didn’t qualify for an involuntary psychiatric hold.
Law enforcement also was involved in an incident in 2015 in which Long was the victim of a bar fight, the sheriff said.
Long was married in 2009 in Honolulu, but separated in 2011 and dissolved the marriage in April 2013 in Ventura County, California.
He and his then-wife said they had no children and owned no property.