"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.
The dead included 11 people inside the bar and a veteran sheriff's sergeant who was the first officer inside the door, Ventura County Sheriff Geoff Dean said.
"It's a horrific scene in there", Dean told a news conference in the parking lot of the Borderline Bar & Grill.
All morning, people looking for missing friends and relatives arrived at a community center where authorities and counselors were informing the next-of-kin of those who died. The official was not authorized to discuss the investigation publicly and spoke on condition of anonymity.
It was the nation's deadliest such attack since 17 students and teachers were killed at a Parkland, Florida, high school nine months ago.
The incident came less than two weeks after a gunman with an automatic rifle fatally shot 11 people during sabbath services at a Pittsburgh synagogue.
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. No other information on the victims was immediately known.
The gunman at the dance bar was tall and wearing all black with a hood and his face partly covered, witnesses told TV stations at the scene.
Matt Wennerstron, a 20 year old college student and regular at the bar, said the shooter fired a short-barreled pistol that apparently had a 10-15 round magazine. He said he thought their injuries were minor, and that majority were likely injured as they escaped, some by breaking windows.
"They felt he might be suffering from PTSD, the fact he was a veteran and had been in the corps", Dean said. "He stepped inside", Mr Dean said.
On Thursday, police identified the deceased officer as Sgt. Ron Helus, a 29-year veteran of the Ventura force.
A sergeant from the sheriff's office was the first police officer to enter the bar and was met immediately by gunfire.
Police found Lindsey and the others holed up in the attic and knocked on the door, telling them it was OK to come out, she said.More news: Tia Mowry Mourns Death of Tamera's Niece in Thousand Oaks Shooting
A sheriff says 13 people are dead, including a sheriff's sergeant and the gunman, after a shooting inside a crowded Southern California bar late Wednesday.
Numerous patrons were drawn by the "college country night" promotion.
On Wednesday evening, there were at least 100 people inside the Borderline bar - which describes itself as Ventura County's largest country dance hall and live music venue.
Screaming in fear, patrons rushed for the exits, dived under tables and used barstools to smash second-floor windows and jump to safety as gunfire reverberated through the Borderline Bar & Grill, a hangout popular with students from nearby California Lutheran University. It's also close to several other universities including California State University Channel Islands in Camarillo, Pepperdine University in Malibu and Moorpark College in Moorpark.
People screamed and fled to all corners of the bar, while a few people threw barstools through the windows and helped dozens escape, witnesses said.
Video accessed by the AP showed law enforcement officers and vehicles speeding to the scene and people running from the bar. A terrified witness runs out.
From inside her auto, she saw a police officer arrive, she said. Three men rush out carrying a bloodied fourth individual. At least one suffered a nonfatal gunshot wound. "Then he shot the cashier, just a young girl". "He was totally committed, he gave his all and tonight, as I told his wife, he died a hero; he went in to save lives, to save other people", he said. That exit leads to a patio where people smoke.
"All of a sudden we heard the "bang, bang" of the gunshots and it just started going insane and people were pushing".
"I am very emotional right now", he said. "He shot the doorman, bouncer ... just a young man", Dominguez told CNN affiliate KABC, choking up. "I saw him point to the back at the cash register and he just kept firing".
Knapp said he has friends who haven't been accounted for.
Sarah Rose DeSon told ABC's "Good Morning America" that she saw the shooter draw his gun.
Taylor Von Molt, 21, said she was line dancing when she "heard what I thought was a balloon pop". "A friend yelled 'Everybody down!' We were hiding behind tables trying to keep ourselves covered".