It was the latest chapter in America's epidemic of gun violence.
Witnesses described a man cloaked in black and wearing glasses as he walked onto a dance floor filled with people.
Wednesday night was college night at Borderline, with the restaurant filled with young people, including some from nearby California Lutheran University and from Pepperdine University in Malibu.
Among the victims was Sgt. Ron Helus, a 29-year veteran of the Ventura County Sheriff's Office, who ran into the building after hearing gunshots.
People gathered near the scene of the shooting, hugging and crying as red and blue lights from police cars flickered in the background. Long was a Marine Corps veteran previously deployed to Afghanistan.
Dean said, "It's a horrific scene in there".
Jason Coffman said he had spoken with his son Wednesday night, as the young man went to the Borderline Bar & Grill with friends, the father said.
"I went and spoke at a Jewish synagogue after the tragedy on the East Coast", Dean said, referring to the mass shooting at the Tree of Life synagogue complex in Pittsburgh, in which an anti-Semitic gunman killed 11 worshipers.
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".
Police said he was a heavily tattooed white male who was not carrying any form of identification, according to NBC News. It was not clear whether he killed himself or was killed by officers, Dean said. Investigators are working out the details of what transpired, Dean added.
Mr Trump said on Twitter that he has been "fully briefed on the awful shooting". Police said hundreds were inside when the shooting occurred. A mental health crisis team determined at that time that he should not be taken into custody, the AP reported.
The massacre has rocked the city of Thousand Oaks - known for its sleepy bedroom community and low crime rates.
"Great bravery shown by police".
The Sergeant was shot several times and was left severely wounded. As an American flag was raised into the sky Thursday, scores of people carrying their own flags gathered outside the hospital to pay their respect to sheriff's Sgt. Ron Helus.
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This story will be updated as new information comes in.