An unnamed 67-year-old woman driving a Jaguar west on Alicia "ran a red light in front of the deputy" as she turned left onto Trabuco, California Highway Patrol officer Aaron Rothberg said.
CHP investigators retrieved and reviewed the deputy's dash cam, he said.
"It actually shows (the deputy) was proceeding on a green light," he said. "The other driver for some reason came into his lane and hit him head-on."
The deputy's speed at the time of the accident has not yet been determined, he said.
The Jaguar driver either stopped or slowed for the red turn arrow, Rothberg said. He said several witnesses corroborated this.
More witnesses are being interviewed this week, he said. A full investigation into the accident is ongoing.
The deputy, the driver and a passerby who suffered smoke inhalation while extinguishing a fire in the sheriff's cruiser were taken to the hospital.