Witnesses described a man cloaked in black and wearing glasses as he walked onto a dance floor filled with people.
President Donald Trump tweeted about the tragedy Thursday morning, thanking law enforcement for their bravery.
Breaking news update: Thirteen people have been pronounced dead, including a police officer and the gunman, after a shooting took place at a country bar in Southern California on Wednesday.
The bar's website says its hosts "College Country Night" every Wednesday.
"I've learned it doesn't matter what community you're in", Ventura County Sheriff Geoff Dean told reporters when asked if he was surprised this happened in Thousand Oaks.
The shooting at the Las Vegas country music festival on October 1, 2017 left 58 people dead and more than 500 injured. Long was a Marine Corps veteran previously deployed to Afghanistan.
Prior to heading to the scene, Helus talked to his wife, as he does several times during his shift, Dean said.
"We all live in a free world here", he said.
An injured man told local television channel KTLA: "We just dropped to the ground, we heard a lot of screaming". The gunman died inside the bar, police said. Kuredjian said that a SWAT team and officers from the Ventura and Simi Valley police departments had converged on the scene. Investigators are working out the details of what transpired, Dean added. But Dean said Long had used an extended magazine - illegal in California - during the shooting, which would enable the weapon to fire more rounds before needing to be reloaded. Helus exchanged gunfire with the suspect, identified by authorities as Ian David Long, before being shot multiple times.
Less than two weeks after the Pittsburgh synagogue shooting that killed 11, another mass shooting has taken a dozen lives in California.
Before hearing of his son's death, Mr Coffman had been frantically driving to hospitals around Thousand Oaks in the USA state of California begging for information.
Sergeant Helus and another officer from the Highway Patrol arrived at the scene of the massacre at 11.23pm last night - three minutes after the 911 call went out. God bless all of the victims and families of the victims.
This story is developing.