The venue in a quiet, upscale residential suburb was hosting an event for college students, with possibly several hundred young people in attendance, Captain Garo Kuredjian of the Ventura County Sheriff's office said.
The gunman shot a security guard standing at the front of the bar before entering the building. The suspect reportedly threw smoke bombs inside the bar.
'The gunman had glasses and a black jacket.
Pictures and video footage show firefighters and first responders flooding the scene after multiple injuries were reported.
In this image made from aerial video, police vehicles line a road in the vicinity of a shooting in Thousand Oaks, California, early Thursday, Nov. 8, 2018.
"It was chaos, people jumping out of windows, people hopping over gates to get out" Steinwender told KABC.
It was college night and country two-step lessons were being offered at the Borderline, according to its website.
"I heard a gunshot, I turned around and I saw him shoot a couple more times", student Teylor Whittler, 19, told CNN affiliate KABC.
Squad cars and helicopters swarmed the area.
Police say the threat is still active and the shooter is still at large after opening fire at the Borderline Bar and Grill.
"I can tell you there are at least six victims inside, including a deputy who was injured". The city has been named in the past as one of the best places to live in Southern California. Kuredjian said he could not confirm whether any of those shot were dead or the severity of their injuries. "This is something that thankfully is very unusual and it doesn't happen very frequently in our neighborhood".