Violent radicalization leading to involvement in jihadi terrorism appears to be highly contingent upon two key factors of what has been termed “differential association,” namely contact with radicalizing agents and pre-existing social ties with other radicalized individuals. This empirical study, which examines all those arrested in Spain for jihadi terrorism activities over the four-year period between 2013 and 2016, quantitatively assesses the importance of these two factors and sheds light on why some individuals radicalize while many more with similar demographic and social characteristics, in the same country, do not. The importance of contact with a radicalizing agent points toward the relevance of ideology in the development of jihadi terrorists, while the significance of pre-existing social ties indicates the relevance of communitarian bonds with local networks, which facilitate terrorist radicalization and recruitment.