Azusa police responded to the scene after receiving several calls at 7:30 p.m. and found the vehicle flipped on its roof, said Lt. Paul Dennis. The crash was reported at San Gabriel Canyon Road and Mountain Laurel Way.
A 12-year-old girl and 7-year-old boy were found dead outside the car. A man and woman, both 32, and a 13-year-old girl were found alive outside the car. All three were taken to area hospitals.
Police have not determined whether speeding or alcohol was involved. Dennis said he could not confirm whether the occupants were related or were wearing seat belts.
He said the car was heading south on the two-lane highway when the accident occurred about half a mile north of the U.S. forestry information booth. It was not clear whether there were guardrails, but Dennis said there was "plenty of distance between the roadway and where the car went down."
Emergency personnel from the Los Angeles County Fire Department, the L.A. County Sheriff's Department and L.A. County coroner's office also participated in the rescue and investigation.
Dennis asked that anyone with information contact Azusa police at (626) 812-3200.