This article analyzes the first investigative steps taken in criminal investigations carried out inside prison This article analyzes the first investigative steps taken in criminal investigations carried out inside prison. The objective is to evaluate whether these proceedings coincide with the functions assigned to each institution and respect national and international procedural norms. In this context, we are interested in knowing what instructions are given by prosecutors; what type of proceedings are carried out; and who carries out the different proceedings. Our hypothesis is that Gendarmería de Chile assumes investigative roles that exceed its penitentiary functions, which generates potentially illicit evidence and leaves the victims ofthese crimes unprotected. The methodology is based on a mixed analysis of 58 reports from Gendarmería (2021-2022), covering homicides, injuries, sexual crimes, unlawful coercion and drug offenses. A quantitative analysis of actions carried out was combined with a qualitative analysis of specific cases. The results highlight that the Gendarmerie assumes key investigative functions, such as guarding the scene and taking statements, without authorization from the prosecutor’s office. In addition, prosecutors often issue generic instructions and do not guarantee protection measures for victims. This contradicts national and international standards, affecting the legality and effectiveness of investigations. It is concluded that it is necessary to improve institutional coordination and guarantee respect for fundamental rights in criminal investigations carried out in prison.