Police said they responded to the Borderline Bar and Grill in Thousand Oaks, about 40 miles west of Los Angeles, just before midnight local time.
All the victims were killed inside the bar in the suburb of Thousand Oaks late on Wednesday, including the officer who had been called to the scene, Sheriff Geoff Dean told reporters.
At least 11 people were injured after a gunman forced his way into a popular local bar and opened fire, according to police attending the scene in Thousand Oaks, California, officials say.
The Borderline Bar and Grill is a country-themed bar known for its large dance floor and live music events, according to its website.
Many more people had more minor injuries, including some that came from their attempt to flee, Dean said.
Speaking at press conference in the wee hours of Thursday, a sheriff said that around a dozen other people had been injured.
Early reports from witnesses suggest that the suspect used a smoke bomb when he entered the bar before he began shooting.More news: RACQ CQ Rescue helps save third shark-attack victim
Dean said around 10 other people were wounded in the shooting. "There were people hiding in attics.It's a horrific scene in there".
The Times reports that there were multiple reports of shots fired and people hit. Great bravery shown by police.
Helus, a 29-year veteran, was one of the first on the scene and was sacked on multiple times.
Sarah Rose DeSon told ABC's "Good Morning America" that she saw the shooter draw his gun. "There were probably 12 shots before I got out the front door".
An unnamed witness told the newspaper that someone ran into the bar around 11:30 pm and started shooting what looked to be a black pistol.
That shooting was politically sensitive: the suspect, Robert Bowers, who said he wanted to kill Jews, argued that a Jewish advocacy group had been aiding a Central American migrant caravan denounced repeatedly by President Donald Trump in the run-up to Tuesday's midterm election.
The shooter took at least 30 shots.