Ventura County Sheriff's Office Capt. Garo Kuredjian said late Wednesday that it was still "a very active scene".
Ron Helus, a 29-year veteran of the department and an officer from the California Highway Patrol were the first to arrive on the scene just before 11:30 p.m. on Wednesday.
A friend, Sgt. Eric Buschow, said Helus took on some of the toughest assignments, was on the SWAT team for much of his career and worked in narcotics and investigations.
"I told his wife he died a hero", Dean said.
"For many of us, it was seeing each others' faces in the bar again that brought us a huge sense of relief", she said in an interview with HuffPost.
Police cars are seen outside a country music bar and dance hall in Thousand Oaks, west of Los Angeles, after a gunman barged into a large, crowded venue and opened fire late November 7, 2018, killing at least 11 people, U.S. police said.
It was "college night" at the bar, and many students were there to learn the country two-step.
"I never thought I would see the things around the country that would happen, but I've learned that it doesn't matter what community you're in, it doesn't matter how safe your community is - it can happen anywhere", Sheriff Geoff Dean said during a press conference.
Authorities believe Ian David Long eventually turned the gun on himself, but not before the 28-year-old, dressed in all black, terrorized a crowd filled with college students at Borderline Bar & Grill in Thousand Oaks, Calif.
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When the shooting was over, 13 were dead, including the shooter who was found by responding officers.
He added that Thousand Oaks "is considered one of the safest cities in the country".
Sgt. Helus had a wife and son, and was due to retire in the next year from Ventura County, California Sheriff's Office.
Officials say the suspect, who has not been identified, was found dead inside.
Authorities do not yet know his name or have any idea of a motive, Dean said.
The shooting began at 23:20 local time on Wednesday about 40 miles (65km) north-west of Los Angeles, police say.
"I saw somebody on the floor as I was running to the front door to get out", he said.
One of whom also survived the Vegas shooting, the LA Times reported. She said she hid with a group of strangers and they were holding hands.
Deputies are at Long's home, seeking to serve a search warrant, Dean said.