Different types of unsolved cases

Unsolved cases is quite a big topic, therefore there are different types of unsolved cases. One of it is murder or homicide cases. This is when the victim was murdered or killed violently and the murderer does not leave any evidence at the crime scene. Besides that, kidnapped cases would be another type of unsolved cases. When there is no signs or leads of the kidnappers or victims, the case is considered to be unsolved. Therefore, it could still be under investigation even though it has been years. Other than that, mysterious cases could also be an unsolved cases. This is when the criminal kill or kidnap their victims and leave their own sign such as "X" mark on their victims, a mask to cover their identity or attacking the victims using the same kind of weapon, for example Axeman of New Orleans who uses an axe to murder his victims. 

Clues that The Zodiac Killer had left.

Mask that frequently used by criminal to conceal their identity.