Editor’s note: A previous version of the video in this story misidentified the victim’s first name. The clip has been updated to correct the error.

(KTLA) — The Los Angeles County Sheriff’s Department is asking for the public’s help in finding a man who shot another man, then chased the victim down and shot him again multiple times while he was on the phone with 911.

Detective James Phillips of the Los Angeles County Sheriff’s Department said the suspect opened fire on an unsuspecting driver, later identified as Jason Castillo, near an intersection in Lancaster, California, in the early morning hours of Dec. 3.

“We definitely believe, with the evidence we have, that during the shooting, they had actually crashed into each other. We are guessing that the suspect was ramming into the victim’s vehicle,” Phillips said.

Castillo, described by investigators as a family man with no criminal history, was wounded in the initial attack and fled to a nearby gas station to call for help, authorities said.

However, the shooter was “driving around looking for the victim,” Phillips said, and once he found Castillo, the shooter carried out what Phillips called “a violent, brutal attack that seems unwarranted.”

The shooter pulled up to the gas station in his white four-door BMW — possibly a 3 series with tinted windows and gray rims that likely has front-end damage due to the crash — got out and opened fire again, surveillance video shows.

“Mr. Castillo was on the phone with 911 and was laying on the ground, explaining to the operator that he had been shot while driving. The suspect approached Mr. Castillo as he laid there and shot additional rounds into his head,” Phillips said.

Castillo survived the shooting, though he remains in critical condition and is unable to speak, authorities said.

The Sheriff’s Department is asking for information to help them find the shooter.

“We definitely tried many avenues, and now we are asking for the public’s help,” Phillips said.

The L.A. County Board of Supervisors and the city of Lancaster are offering a $20,000 reward for the arrest and conviction of the suspect. At the time of the shooting, he was wearing a green T-shirt, black shorts, white socks and a black baseball hat.

Detectives from the Los Angeles County Sheriff’s Department Operation Safe Streets Bureau are handling the case. Anyone with information is encouraged to call Detective James Phillips at 661-948-8466 or 310-601-6064.

To provide information anonymously, contact Crime Stoppers by dialing 800-222-TIPS (8477) or visit lacrimestoppers.org.