A theoretical model of genesis and development of clandestine terrorist communities is offered in the article. Proceeding from social ontologies’ typology worked out by V.M.Sergeyev and N.I.Biryukov, as well as from social nets’ and hierarchies’ interrelations model proposed by V.M.Sergeyev and A.S.Kuzmin, the author formulates two hypotheses. In accordance with the first one, clandestine net communities arise under the conditions of: (a) lack of the socium’s ontological, value and institutional integration and, as a result, formation of “grey” zones, (b) delegitimation of the official state hierarchy, (c) predominance of the dualistic type of ontology, (d) low vertical mobility within the dominating hierarchy and (e) availability, in society, of active people notable for community of ontology and of value orientations. According to the second hypothesis, terrorist organizations grow from movements and pass in their evolution four stages: movement — (hierarchically structured) social net — hierarchy — several hierarchically structured social nets. In order to verify and substantiate the hypotheses put forward, the author turns to analyzing the experience of one of the most notorious terrorist organizations of Western Europe in the second half of the 20th century — the Italian “Brigate rosse”.