Terrified patrons scrambled for the exits after the gunman started shooting a pistol in the Borderline Bar and Grill around 11:20 pm on Wednesday before killing himself, they said. "To save other people".
The Southern California city, situated 40 miles northwest of Los Angeles, is an upscale community in the foothills of the Santa Monica Mountains with a population of almost 130,000 and a median household income of $101,045.
Police have identified the suspected killer as 28-year-old Ian David Long, a former Marine.
Dean said Long was a Marine veteran, and said there was a chance he suffered from PTSD.
"He had ideal form", bar patron Teylor Whittler told Fox News.
The mother of the 27-year-old man killed in the latest attack, Telemachus "Tel" Orfanos, said her son survived Vegas only to die inside Borderline, less than 10 minutes from his home.
Authorities so far had no indication of a motive, or of any connection to terrorism.
They have not yet confirmed any motive behind the attack.
Media captionSurvivors of California shooting describe what they saw.
Soon after the shooting started, Sergeant Ron Helus, a veteran of 29 years, was first on the scene and entered the bar with another policeman.
The Ventura County Sheriff's Department said 21 people had been treated for injuries and released at area hospitals.
Ron Helus was among those killed, authorities said.
Dean says the report of the shooting came as Helus was on the phone with his wife. Speaking to reporters Thursday, Jason Coffman said he talked to his son Cody on Wednesday night before he headed out the door. "It sounded like fireworks or something", said witness John Hedge, according to NBC News.
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"We all just trampled and tried to get out", Tait said. Investigators didn't immediately know how many rounds the magazine could hold, he said. "I could still hear gunshots after everyone left", the Times quoted the man as saying. I look up. The security guard. was shot, he was down.
Dean described the scene inside the bar as "like hell".
Officers went to Long's home in Newbury Park, about 4 miles (6 km) from the bar, in April to answer a disturbance call and found him agitated, Dean told reporters.
Earlier, on "CBS This Morning", Dean said he and Helus were friends. A mental health specialist met with him and didn't feel he needed to be hospitalized.
Police believed Long fired around 30 shots with a Glock 21 handgun with an extended magazine created to hold extra bullets - something that is illegal in California.
People wait in line to donate blood outside LaReina High School after a mass shooting by a lone gunman at a bar in Thousand Oaks, Calif., Nov. 8, 2018.
Knapp said he helped people hide behind a pool table and then fled outside, alerting people on an outdoor smoking patio and helping carry a victim to an ambulance.
Many of those at the bar were regulars, she said.
They found 12 people dead inside, including the gunman.
A next-door neighbour said he called authorities about six months ago when he heard loud banging and shouting come from Long's ranch-style home.
Marc Orfanos said that he blamed the country's "gun culture" for the prevalence of mass shootings. A sheriff's deputy, 12 attendees and the gunmen were killed.
In a tweet, President Donald Trump applauded the response of police and extended blessings to the victims and their families.
Trump ordered USA flags to be flown at half-staff at public buildings and grounds.