Officials say the suspect, who has not been identified, was found dead inside.
California governor-elect Gavin Newsom (D) lamented the country's "gun culture" after the latest mass shooting.
That, in turn, closely followed the series of pipe bombs mailed to critics of President Donald Trump.
Law enforcement agencies, firefighters and other first responders, along with several ambulances, additional units from the California Highway Patrol, Simi Valley and the Federal Bureau of Investigation, reached the spot soon after the shooting was reported. There's a fence right there so I said: "'Everyone get over the fence as quickly as you can.' And I followed them over".
When the gunman stopped to reload, he and others grabbed barstools and used them to smash open a window.
Helus was immediately hit with multiple gunshots, Dean said.
By the time they entered the bar again the gunfire had stopped. As previously reported, Long, who was found dead at the scene from a presumably self-inflicted gunshot wound, was a veteran of the Marines who served in Afghanistan from 2010 to 2011. It's not yet clear how the gunman died, Dean said. Two-step lessons in country dancing were being offered Wednesday at the Borderline, according to its website.
California Lutheran University canceled classes Thursday following the shooting.
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"He was shooting everything, from the speakers to anything", Kayla Simmons said. Then he looked up and saw a man in a black hoodie holding a gun.
"There are a few people we can't get a hold of, but in these situations, people usually turn off their phones to be safe so I'm not gonna get too anxious. You're always over-analyzing people, trying to figure out if something were to go down, 'What would I do?'" That exit leads to a patio where people smoke. "It probably went on for at least 10". "I would open the garage and would be like, 'What are you doing?'"
Mourning: A procession for the body of Sergeant Ron Helus, who died in the shooting, drives down Ventura Highway 101. "I would make fun of him, because he would drag me there".
Knapp said he has friends who haven't been accounted for.
The officer was on the phone to his wife when the emergency call came in, telling her: "Hey I gotta go handle a call. I want gun control", she said to reporters.
The video then showed Mr Knapp fleeing through a door and running down a street, shouting that there were still people trapped in the club in the city of Thousand Oaks.
Offering a horrifying illustration of just how pervasive gun violence is in America, Susan Orfanos said her son survived the Las Vegas festival shooting last October that left over 50 people dead-making it the deadlist mass shooting in modern USA history.