This document analyses two types of homicide: those where the authorities managed to determine the reason for homicide, and those registered as ‘homicides without known reason’. The aim is to understand and explain the factors that come together in a criminal investigation and which contribute to classifying a homicide event as ‘known’ or ‘unknown’. Our findings indicate that age (< 10 years), place (indoor) and use of substances (drugs) are key elements to explain how homicide investigations are prosecuted by the criminal justice system in Mexico.