The teen, identified by her family as Hannah Williams, was shot Friday during the incident involving an on-duty Fullerton police officer.
The Orange County District Attorney's Office is now investigating the shooting and says it began when the officer observed a teenage driver traveling at a high rate of speed on the freeway.
At some point, the two vehicles made contact and the officer-involved shooting occurred. The girl was transported to the hospital where she was pronounced dead.
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Eyewitness News spoke to one witness who said she believes the officer had to shoot.
"She brought her arms up with a gun. Now, I don't know why. I don't know why. He had his gun out, you can see it clearly. He was yelling commands at her but she didn't listen," said Renay Arnold.
The replica handgun found at the scene is designed to look like a real Beretta 92 FS.
Meanwhile, family members of the teen are calling for the Fullerton police officer to be suspended without pay until the department's internal investigation is finished.
At a press conference alongside civil rights activists Tuesday, they also called on California Gov. Gavin Newsom to direct Attorney General Xavier Becerra to conduct a separate independent investigation.
The family contends that Williams was driving a rental car and they don't know who the gun belonged to.
They also said they intend to have an independent autopsy performed once they gain custody of the body.