The attack was the second mass shooting in the U.S.in less than two weeks.
Once again, President Donald Trump has ordered flags at half-staff "in a mark of solemn respect".
Ventura County Sheriff Geoff Dean says the suspected gunman died at the scene from what they believe is a self-inflicted wound.
Reports say the suspect may have used smoke grenades and at least one weapon, described as a semi-automatic handgun by witnesses.
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.
Sgt. Ron Helus had been set to retire in the next year from the Ventura County, California Sheriff's Office when, responding late Wednesday to a call of a shooting, he walked into the Borderline Bar & Grill.
Kuredjian said it has been "quite some time" since there was a shooting of any kind in Thousand Oaks, a city of about 130,000 people about 40 miles (64 kilometres) west of Los Angeles, just across the county line.
"The fact that he was the first in the door doesn't surprise me at all, " sheriff's Sgt Eric Buschow said Thursday.
Authorities said he had no criminal record, but in April officers were called to his home, where deputies found him angry and acting irrationally.
"There were people in the middle dancing and just hanging out and having a good time and you hear that and you just know something's up", Erika Sigman said. He was identified as Sheriff's Sgt. Ron Helus, a 29-year veteran about a year from retiring. He is survived by his wife and son. "There's no way to describe this". "We go to the gym together, work out together.it's horrific and bad and it saddens our hearts". He died a hero. "He went in to save lives, to save other people". The identities of the other victims have not been released at this time.
"He was somewhat irate, acting a little irrationally", Sheriff Dean said, but he said he was unaware of a PTSD or mental illness diagnosis.
One reporter asked the Sheriff what could be done to stem the flow of mass shootings in the United States. They are still searching and investigating. "Things are a little sketchy - we don't have a lot of information right now". He added that they have no reason to think there was a link to terrorism, "but we certainly will look at that option".
Federal authorities also responded to the scene, including agents for the Bureau of Alcohol, Tobacco, Firearms and Explosives.
There were roughly 200 people in the bar for a popular weekly "College Country" night, police said.