Witnesses described a chaotic scene as panicked patrons smashed windows to get out.
There was no known motive, he said.
Among the victims was Cody Coffman, his father confirmed to CBS Los Angeles on Thursday.
The extent of the victims' injuries are now unknown after the shooting at the student-filled Borderline bar and grill.
The bar's website says it hosts College Country Night every Wednesday. Someone yelled "Get out - he's coming", and she fled as the gunman fired another round of shots.
"He was just a great guy, a gentle soul, " Buschow said. He said she told him the shooter threw tear gas into the bar and then she heard gunshots and ran out the back. I was only there for a good 10 minutes but I just - there's like no words. The police officer who was shot and killed was identified as Sheriff's Sgt. Ron Helus, who was labelled a "hero" for attempting to stop the incident.
He joined the Marines in 2008 and rose to the rank of corporal before leaving active service in 2013, Marine spokesman Joseph Butterfield wrote in an email.
Ian David Long served in the Marines for almost five years, including a seven-month tour in Afghanistan.
He was interviewed by police at his home in April after an episode of agitated behaviour that they were told might be post-traumatic stress disorder.
"It's so tragic losing Ron", Dean said. But it had an extended magazine - one capable of holding more ammunition - that is illegal in California, Dean said.
"Obviously he had something going on in his head that would cause him to do something like this."They called a mental health specialist who assessed Long but concluded he couldn't be involuntarily committed for psychiatric observation. "We haven't found any correlation", the sheriff said. He said he had friends who hadn't been accounted for.
Officers escorted large groups of people to safety as they left the area.
A woman in the background added: "He was kind of a loner".
The body of a veteran sheriff's sergeant fatally shot while responding to a Southern California mass shooting is being taken in a procession from a hospital to a coroner's office.
Helus was among the first officers through the door at the Borderline. He says Helus was the first responder and was immediately hit with multiple gunshots. The suspect was later found dead inside the bar.