It happened Saturday around 3 a.m. along the northbound lanes of the 5 Freeway at Weldon Canyon Road.
The California Highway Patrol said the girl fell out from the back of a 1999 Lincoln Navigator that was being driven by her mother, who's been identified as 28-year-old Veronda Gladney.
Investigators said multiple vehicles slammed into the girl moving at freeway speeds.
"[The girl] was in the very back portion of the 1999 Lincoln Navigator," said CHP Ofc. Joshua Greengard. "There was witnesses saying that she was either hanging out or playing in the back portion, the very very back portion of it. That window was already broken out, and it looks like she may have fallen out of the vehicle and then tragically died."
Greengard said the girl was not strapped into a child seat when she somehow fell out of the broken back window.
Gladney's boyfriend, Michael Branch, was also in the SUV. The two have been arrested for child endangerment, according to authorities. Branch was released on his own recognizance by a judge Wednesday afternoon.
Formal charges have not been filed.
Greengard said in the 14 years he's been with CHP, he's never seen such a tragic accident.
"I've never had a kid falling out of a moving vehicle," said Greengard. "I've been to numerous tragedies in my time, but this one is very unfortunate. It's very sad."
California law states children under the age of 8 must be secured in a car seat or booster seat in the back seat of a vehicle.
Children who are 8 or have reached 4 feet 9 inches in height may be secured by a booster seat, but at a minimum, must be secured by a seat belt.
Greengard said witnesses - including drivers who hit the child - were all distraught.
He said investigators interviewed Gladney, who was also in pain.
"From other officers, I know she was distraught also," said Greengard.
This incident remains under investigation and anyone with information is being urged to contact the CHP Newhall Area Office at 661-600-1600.