Police said that hundreds of people were in the Borderline Bar & Grill in Thousand Oaks and shots were still being fired when officers arrived.
There were reports of pools of blood in the area, and responders attending to one injured person outside the venue.
"They found 11 victims that had been killed", said Dean of the first response unit before detailing that the death of Helus brought the toll to 12, not including the gunman.
They have not yet confirmed any motive behind the attack. Nine months ago, another mass shooting occurred in Parkland, Florida, where 17 students and teachers were fatally shot at Marjory Stoneman Douglas High School.
The sheriff says he doesn't remember the city having another mass shooting in his time on the force.
He said the man set off smoke grenades inside before opening fire.
The anxious father said the tracking device on the young man's phone showed it was still at the club, and "It's not moving. My friend is the DJ so she cut off the music, we just heard a lot of mayhem". We are still looking for the shooter. This is a heart I will never get back.
"There were people hiding in restrooms, people hiding in attics", Dean said.
"I've learned it doesn't matter what community you're in", Dean told reporters when asked if he was surprised this happened in Thousand Oaks.
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The husband and father-of-one had been a policeman for 29 years and was due to retire next year.
"Losing Ron, we go to the gym together and work out together", Dean said. He died a hero.
Helus succumbed to his wounds at a hospital, police said. It is believed he took his own life. "I don't know what to tell other people", Coffman said, his head hanging.
Also, both the Vegas and Borderline shootings involved country music.
"It was chaos, people jumping out of windows, hopping over gates and just trying to get out", he said.
Three of the deadliest mass shootings in United States modern history have occurred since October 2017.
Authorities have identified the gunman and the handgun he used to kill 12 people at a southern California bar. On Oct. 27, a gunman killed 11 people at a synagogue in Pittsburgh.
According to the website Gun Violence Archive, more than 12,000 people have been killed using firearms in the USA so far this year, including about 3,000 people under 18.