The Pacific County prosecutor's office, which also handles coroner duties, said Monday that Banh drowned accidentally.
The sheriff's office said she parked on the beach Sunday and told her 8-year-old daughter to stay inside while she went fishing.
Someone walking on the beach Sunday afternoon found the body and the car with the child inside. The car had been surrounded by water from the rising tide.
The girl was not injured. She has been placed with her grandparents.