Bruising and lacerations are easy to identify. Internal injuries and bleeding are much harder to see and often require a diagnosis from a medical professional using specialized diagnostic equipment.
Internal injuries and bleeding can start after the accident and continue while the victim is in the ambulance or emergency room. And such injuries can sometimes go on for an extended period of time without being noticed.
Internal bleeding can happen quickly, and if the bleeding is serious enough, the fatality may occur before medical treatment can be administered.