The two-vehicle collision was reported shortly before 5 p.m. Thursday at 220th Street and Western Avenue, according to the Torrance Police Department.
According to a preliminary investigation, police said, the cars collided as one of the drivers was trying to make a left turn onto 220th Street from Western Avenue.
The baby and her mother were passengers in the back seat of the turning vehicle; they were ejected upon impact and suffered serious injuries. Officers and firefighter-paramedics rushed the two victims to Harbor General Hospital.
The infant was later pronounced dead at the medical center, police said in a statement. The mother remained in critical condition on Friday.
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Neither of the two victims were publicly identified, "pending notifications and respect for the deceased 5-month-old's family," the news release said.
No arrests or citations have been announced in the connection with the crash, which is under investigation.