In this article we study the organizational structure of a large group of members of the Sicilian Mafia by means of social network analysis and an econometric analysis of link formation. Our mains results are the following. i) The Mafia network is a small-world network adjusted by its criminal nature, and is strongly disassortative. ii) Mafia bosses are not always central in the network. In particular, consistent with a prediction of Baccara and Bar-Isaac, we identify a "cell-dominated hierarchy" in the network: a key member is not central, but is connected to a relative with a central position. iii) The probability of link formation between two agents is higher if the two agents belong to the same Mandamento, if they share a high number of similar tasks, while being a "boss" reduces the probability of link formation between them. iv) The probability of link formation for an individual agent is higher if he is in charge of keeping connections outside his Mandamento, of collecting protection money and or having a directive role, while age has modest role. These results are interpreted in the light of the efficiency/security trade-off faced by the Mafia and of its known hierarchical structure.