It seems that despite the passage of time the wave of homicides followed by suicides of perpetrators has not decreased and the problem remains not fully explained or explored. From the point of view of the law enforcement every such case requires a forensic scene of crime examination by experts of various specialties before assessing it in the context of criminal law. In Polish professional literature such cases are described as “extended” or “post-aggression” suicides. The hereby article is an attempt to systemise terminology used in this area by pointing out fundamental differences between two notions, which are not synonyms. A “postaggression suicide” is a wider, descriptive and neutral term because it indicates an undetermined act of aggression preceding an attempt to take one’s life. On the other hand, an “extended suicide” despite the suggestion that several persons might have committed suicide at the same time, in fact defines an act by the perpetrator (killer) conditioned by his/her psychological state projected on the other persons. The perpetrator attributes own sufferings to the victims and by killing them, in a sense, frees them. Therefore, children and, less frequently, spouses and, in general, persons the perpetrator has positive feelings for fall victims of an extended suicide. The motive of the killing is protecting of close persons from suffering, misfortune or difficult life situation, which the perpetrator is experiencing. A clear distinction between an extended suicide and postaggression suicide is of great practical significance because it determines, as emphasised by the Authors, the course of investigative proceedings, decisions at pre-court stage, as well as the extent of forensic examinations requested from forensic specialists.