A motorist who said she was being followed in traffic was fatally shot by the alleged stalker after she pulled over to seek help from a cop parked on a Hawthorne street yesterday, authorities said.

The brazen 12:30 p.m. attack happened in front of a Hawthorne Police Department officer the woman had contacted for protection, cops said. Sheriff's Lt. David Coleman told reporters, “The right front passenger of the suspect vehicle got out and started firing … in front of the police officer.”

A 12-year-old who was in the victim's car was hit by gunfire and fled before he was found and hospitalized in stable condition, authorities said.

The officer who was trying to help the woman immediately opened fire on the suspect, killing him on the spot, according to the L.A. Sheriff's Department, which has been assigned to investigate the case.

The cop was still sitting in his patrol car, which he had stopped to monitor traffic at Rosecrans and Kornblum avenues, when he waved the victim over. Gunfire erupted soon after the car the suspect was in rolled up, according to a Sheriff's statement.

The woman was in a late-model Mercedes E-Class coupe. The suspect appeared to be in a white, '90s-era Cadillac sedan, according to our look at news helicopter footage of the scene.

Two other men in the alleged stalker vehicle, the driver and a rear-seat passenger, were detained by police and questioned, deputies said. It's unclear what the relationship was, if any, between the shooter and the woman, they said.

Here's how sheriff's officials said it all went down:

 … A Hawthorne police officer was parked southbound on Kornblum Avenue, in his marked patrol vehicle, monitoring traffic when the victim pulled her vehicle adjacent to his and told him that another vehicle had been following her. The officer told her to pull her vehicle in front of his and pull over so that he could assist her. At that moment, another vehicle pulled adjacent to her. A male adult exited that vehicle with a firearm and began firing at the female and her passenger, a male juvenile. The Hawthorne police officer exited his patrol vehicle and fired his weapon at the suspect. The suspect was struck by gunfire and was pronounced dead at the scene.

In addition to Sheriff's detectives looking into the events, the L.A. County District Attorney's Office and the Hawthorne Police Department have investigators conducting their own inquiries.

The sheriff's department says:

The members of the Hawthorne Police Department extend their prayers and wishes to the young boy struggling to survive this horrific assault. Anyone with information about this incident is encouraged to contact the Los Angeles County Sheriff's Department's Homicide Bureau at (323) 890-5500 or the Hawthorne Police Department at (310) 349-2700.

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