The Ventura County Sheriff's Office told CNN that there was a shooting at a bar, but declined to release additional details.
He was one of the first law enforcement officials to respond - and called his wife just before walking into the bar where he was shot dead.
The killer was identified as 28-year-old Ian David Long, a former machine gunner and veteran of the war in Afghanistan who was interviewed by police at his home last spring after an episode of agitated behaviour that they were told might be post-traumatic stress disorder. The sheriff believes Dean took his own life.
Long had been cleared by mental health experts after an incident in his home in April, sheriffs revealed on Thursday morning.
The Los Angeles Times quoted a law enforcement official as saying at least 30 shots had been fired.
President Donald Trump ordered the United States flag at the White House and other government buildings to be flown at half-staff following the shooting, which came just 10 days after a gunman killed 11 worshippers at a synagogue in Pittsburgh. He lives near the bar and said numerous regulars grew up in the area and have known each other since they were kids.
Ron Helus, a Ventura county sheriff's sergeant, was among those killed at the Borderline Bar and Grill, about 40 miles west of Los Angeles.
Survivors described moments of panic and heroism.
"It took a couple of seconds for people to realize what was going on and once that happened it was just utter chaos", he said.
Erika Sigman, 19, was another witness. She said she hid with a group of strangers and they were holding hands.
When the Las Vegas gunman opened fire from a 32nd-floor hotel room, Kelly said he threw a friend to the ground before helping get her out of the area and into a room.
The one-hour procession through the streets was filled with sadness and silent salutes on numerous overpasses. A reporter spoke to a man who had not heard from his son. He said he had an army of friends and families on alert, making calls, and watching the news looking out for his son.
Von Molt said she and others from nearby California Lutheran University often gathered on Wednesday night to line dance to country music. At the time, police said Long "was somewhat irate, acting a little irrationally".
At an earlier news conference Sheriff Dean's voice cracked as he spoke about Sgt. Helus, "He went in to save lives. He went in to save other people".
"Obviously he had something going on in his head that would cause him to do something like this", Dean said.
After a lull of about 15 minutes, more officers went inside and found the suspected gunman dead of a gunshot wound.
Dean told reporters that Long shot an unarmed security guard who was standing outside the bar before entering and opening fire at random inside.
Reports said there were around a 100 patrons inside the nightclub when the shooting began. The bar is near the campus.