While most of the victims are yet to be identified, Ventura County Sheriff Geoff Dean revealed one of the victims was a 29-year veteran of the force, Sgt. Ron Helus. "As far as I know, all of my friends are OK, safe", he said.
A police officer, described as a sheriff's sergeant, and the gunman are among the 13 confirmed to have died.
Authorities urged the public to avoid the area.
The US has suffered from mass shootings for decades, but most of the deadliest have occurred in recent years.
The previous most deadly attack was another just a year earlier, when 49 people were killed in Pulse - a gay nightclub - in June 2016. in Orlando.
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.
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The gunman was tall and wearing all black with a hood and his face partly covered, witnesses told TV stations.
"When the officers went in and made re-entry, they found him already deceased", Dean said, according to ABC 6.
She said she had planned to go with them but stayed behind to do some schoolwork instead.
He later died in hospital.
By the time they entered the bar again, the gunfire had stopped, according to the sheriff.
Friends are remembering a USA sheriff's sergeant who was killed trying to stop a shooting rampage at a California bar as a "cop's cop" who didn't hesitate to run toward danger.
"He died tonight going in to try to save others and paid the ultimate sacrifice", Dean said.
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"Ongoing active shooter incident reported at Borderline", the Ventura County Fire Department tweeted.
Americans woke up to another mass shooting tragedy on Thursday morning, this time in Thousand Oaks, California at a popular college student spot, Borderline Bar & Grill, which was hosting its weekly 18-and-over College Country Night. It is also close to several other universities, including California State University Channel Islands in Camarillo, Pepperdine University in Malibu and Moorpark College in Moorpark. "I'm sorry for their loss", said the tearful father as he stood next to his father-in-law Mike Johnston.
The last words Jason Coffman told his son, Cody Coffman, were "I love you", before he went to Borderline Bar and Grill.
Whittler said during her escape she was trampled on and hit in the head with a barstool that was being thrown at a window by another person trying to get out.
"We don't know who shot anybody at this point".
University President Chris Kimball said Mr Meek "heroically saved lives in the incident".
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Borderline Bar and Grill, a popular country venue, was hosting its weekly college night when the attack occurred on Wednesday. The bar shooting was the latest mass killing in the United States and was sure to revive the debate on gun control.