The unnamed gunman opened fire at the Borderline Bar and Grill around midnight on Wednesday, Nov. 7, injuring at least 11 people.
Helus died at about 2 a.m.at Los Robles Hospital in Thousand Oaks.
The gunman also died at the scene, though it was not immediately clear how.
He had other interactions with law enforcement, including a traffic accident and an incident when he was the victim of a battery at a bar.
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This was the deadliest mass killing in the US since the massacre in Parkland, Florida, on February 14, 2017.
The married father-of-one was among 12 people to have been killed by the 28-year-old US Marines veteran, who is believed to have later turned the gun on himself.
The suspect, who has not been identified, was found dead inside the bar, Dean said.
"All I did was grab as many people as I could and pull them underneath the table until I heard a break in the shots, and then we got people out of there, as much as we could", Wennerstrom told the news station about the moment shots rang out.
Mr Helus was a 29-year veteran of the force with a wife and son and planned to retire in the coming year, said the sheriff, who choked back tears several times as he talked about the sergeant who was also his long-time friend.
Located around 40 miles west of Los Angeles, Thousand Oaks was recently ranked as one of the country's safest cities.
"We are consistently ranked one of the highest with respect to the lowest crime rate per capita, we're proud of that", Fox said. "It's still very fluid, and I hope to give you guys updates in terms of both the number and the status of the shooter".
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"Firefighters and first responders are arriving on scene of a report of a shooting at an establishment in @CityofTO".
Tayler Whitler, 19, said she was dancing when the gunman opened fire.
Long was armed with a Glock 21, a.45-caliber created to hold 10 rounds plus one in the chamber, according to the sheriff.
Dean says the report of the shooting came as Helus was on the phone with his wife.
"(He) said to her, "Hey I got to go handle a call.
Witnesses told ABC News that a man fired several shots from a handgun before tossing smoke bombs and starting to fire again. "That's the stuff I'm truly going to miss", he said.
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