At a morning press conference, Ventura County Sheriff Bill Ayub said Sgt. Ron Helus was struck five times by bullets from the suspect and a sixth time by a round from the CHP officer's rifle.
The suspect killed 12 victims before he fatally turned his weapon on himself.
Investigators have yet to announce 28-year-old Ian David Long's motive for the shooting. Long was carrying seven high-capacity, 30-round magazines when he stormed into the popular venue on the evening of Nov. 7, the Sheriff's Office said.
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The shooter was armed with a folding knife and a Glock .45 caliber semi-automatic pistol he legally bought in 2016 at a Southern California gun shop, according to Ayub.
He threw several smoke grenades and fired over 50 rounds during the rampage, hitting 13 people, including the sheriff's sergeant.
Helus, a 29-year veteran of the Sheriff's Office, was wearing body armor when he was shot, authorities said.
PHOTOS: Mass shooting at nightclub in Thousand Oaks, California