We propose that all evidence collected on a crime scene should be preserved by the police for at least a hundred years.
In cold cases it is not uncommon that collected evidence has been destroyed, once the case is reopened at a later moment. This causes frustration by both investigators and victims (or their kin), because many case will never be solved since crucial evidence is destroyed.
Evidence should also be preserved for a hundred years, even if someone has been convicted for the crime. It is always possible that there has been a miscarriage of justice, and hence it would be desirable to be able to reevaluate evidence during a revision procedure.
The hundred-year period has been chosen because it is longer than an average human life. If a case has not been solved after this period, it is very unlikely that the perpetrator is still alive.