Seat belt defense in car accident

People are dead from a serious car accident on I-71 and all the media can talk about is the failure of the deceased to wear seatbelts. How do they know that a seat belt would have prevented an ejection from the car. Or would have prevented the deaths

It was reported that 19 year old Danica McClester lost control of her car and it rolled . In fairness the actual report I saw stated;