The obligation to prosecute the perpetrator of any crime throughout the entire period of its punishability is one of the statutory tasks of the Police. The prominent forensic scientist — Hans Gross claimed that crime is not an abstract and theoretical entity out of touch with reality, but a real social phenomenon that can be investigated and recognised. Nevertheless, archive shelves contain records of undetected crimes from the past years that cast a shadow over police statistics and never give investigators any peace of mind. These undetected crimes from a few, a dozen or even several dozen years ago, are reinvestigated by officers from the Cold Case Units (colloquially referred to as the Police X-Files). Currently, such a unit operates in each regional police headquarters as well as in the National Police Headquarters. The methods and means that the X-Files investigators take advantage of depend on the specificity of an individual case, however, in the model of their conduct, it is forensic science that plays a significant role — alongside covert policing or criminal analysis. Physical evidence plays a huge role in determining the objective truth, and thanks to the research methods and tools currently available to crime scene investigators, the boundaries of learning about the reality are expanding. Despite the passage of time, the purpose of criminal proceedings is achieved, i.e. perpetrators of many crimes committed years ago are identified and brought to justice.