The police officer was injured when he arrived at the scene with colleagues and "attempted to neutralize the threat", Ventura County Sheriff's Office Capt. Garo Kuredjian said.
At least a dozen people were killed - including a "hero" cop who was due to retire next year and died bravely trying to save others' lives. Add Thousand Oaks Shooting as an interest to stay up to date on the latest Thousand Oaks Shooting news, video, and analysis from ABC News.
"A true cop's cop", Ventura County Sheriff's Sgt. Eric Buschow told CNN Thursday morning.
The sheriff's department had several previous contacts with Long including a call to his home in April when deputies found him acting irate and irrationally.
As the entered, Sergeant Helus was struck multiple times with gunfire.
Sergeant Ron Helus is among the victims killed.
The wounded police officer was taken to hospital, but later died of his injuries.
Further, the officer says a highway patrolman pulled out Helus and waited for a SWAT team as scores of other officers converged on the Borderline Bar & Grill in Thousand Oaks at about 11:20 p.m.
Survivors described moments of panic and heroism.
Authorities are investigating reports that the gunman used a smoke bomb but said they had not found anything to support this.
The sheriff said the gunman has not been identified and there was no known motive for the shooting.
Tayler Whitler, 19, said she was on the dance floor with her friends nearby when she saw the gunman shooting and heard screams to "get down".
Erika Sigman, 19, was another witness.
"He went in there to save people and paid the ultimate price", he said. "It's a big thing for us", he said, explaining he was with several people who were also with him in Las Vegas. Hours into the tragedy no one had been reunified with a loved one. 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.
The attack on the bar - a college favorite and was hosting a country music event - was the nation's deadliest shooting since 17 students and teachers were killed at a Parkland, Florida, high school nine months ago.
People gathered near the scene of the shooting, hugging and crying as red and blue lights from police cars flickered in the background.
Helus was not only a 29-year sheriff veteran, but he was also a husband and father. The bar is near the campus.
"Cal Lutheran wraps its arms around the Meek family and other families, and around every member of this community of caring", said an alert from the university which invited the campus community to join vigils on Thursday.