The monograph examines the features and results of the official investigation into the case of secret societies and the Decembrist conspiracy. For the first time, in addition to the traditionally studied 120 convicted by the Supreme Criminal Court, special attention is paid to a number of other categories of participants in secret societies involved in the process - pardoned, ignored, acquitted by the investigation. The course and results of the investigation in relation to each category are highlighted, the reasons are revealed due to which the figures of the Decembrist conspiracy were forgiven, acquitted and escaped punishment, the main methods and techniques of protecting those under investigation are reconstructed. The author analyzes sources containing information about persons who were not involved in the process and remained unknown to the investigation as participants in secret societies and the conspiracy of 1825. Based on criticism of historical evidence, the names of previously unknown and alleged figures of the Decembrist conspiracy who were not affected by the official investigation are introduced into scientific circulation . The appendix publishes a generalized reference index of all currently known members of the Decembrist secret societies, participants in the conspiracy and military performances of 1825–1826. For historians, legal scholars, and a wide range of readers interested in problems of Russian history and culture.