The killer was identified as Ian David Long, a 28-year-old former USA marine with mental health issues, with investigators trying to find the motive for the attack at a venue hosting a country music evening for students.
The police officer was injured when he arrived at the scene with colleagues and "attempted to neutralize the threat", Ventura County Sheriff's Office Capt. Garo Kuredjian said.
Ventura County Sheriff Geoff Dean said during a news conference Thursday morning that his department has had "several contacts with Mr".
Mental health specialists talked with Long and found that no special action was necessary.
Dean said the other prior encounters were a traffic accident and an incident when he was the victim of battery at a bar.
What could not be counted on would be a gunman, armed with a Glock 21.45-caliber handgun, to begin indiscriminately firing into those throngs of happy people.
"There's no doubt that they saved lives by going in there and engaging with the suspect", said Mr Dean, who was set to retire today. The ATF is now investigating how and when the weapon was purchased.
"Hope has sustained communities, very much like Thousand Oaks, through the exact same triages of mass shootings", said Andy Fox, the city's outgoing mayor.
Some people were so terrified, they asked officers to confirm it's not the gunman.
"It took a couple of seconds for people to realize what was going on and once that happened it was just utter chaos", he said. Neighbor Tom Hanson had called the police with concerns about Long's behavior. Then, Knapp said, he heard gunfire and saw a young woman at the counter shot repeatedly. He shot a security guard outside the bar and then went in and took aim at employees and customers, authorities said.
"First thing I said was, "please don't drink and drive". Police said hundreds were inside when the shooting occurred.
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The 28-year-old was on active duty with the marines from August 2008 to March 2013, CNN reported, citing the Department of Defense.
The man suspected of killing 12 people in a bar in a Los Angeles suburb was a decorated Marine Corp machine gunner deployed in Afghanistan who had several prior brushes with law enforcement, police and Marine officials said on Thursday (Nov 8). He was assigned to the 2nd Battalion, 3rd Marine Regiment, Third Marine Division in Kaneohe Bay, Hawaii.
He served in the war in Afghanistan for seven months beginning in late 2010, the Marine Corp said, and was awarded multiple ribbons, commendations and medals.
Mourners outside a reception centre for families of victims.
Long's cousin, AJ Schramm, told CBS News Long's father passed away from cancer early in his life. The sheriff said it was possible that Long had suffered from post-traumatic stress disorder.
Schramm said Long's mother was "venting" at a recent family weddings about issues she was having at home with Long.
When the gunshots paused, Wennerstorm said he knew the shooter must be reloading, and he didn't waste the moment.
Von Molt said she and others from nearby California Lutheran University often gathered on Wednesday night to line dance to country music.
On Thursday, 12 people including a cop were shot dead in a shooting at a night club close to Los Angeles.
The bar is located in Thousand Oaks, Ventura County, about 40 miles northwest of Los Angeles.