The linkages between organized crime and terrorism are articulated by some scholars interested in explaining how, and the extent to which, organized criminal groups and terrorists/terrorist groups interact. The following section outlines a number of key theoretical models that have been developed by academics in this field and which have application to the study of linkages between organized crime and terrorism. It should be noted that these are only a few of the theoretical models posited by scholars; other models, such as those developed by Hutchinson and O'Malley (2007) and Ballina (2011), are also worth considering when providing a comprehensive theoretical framework of the linkages between these two phenomena.
Williams developed one of the earliest works on the crime-terror nexus theory and identified three distinct hypothetical models of interaction between organized crime and terrorism (Williams, 1998). The first of these models is the "convergence" thesis, which concerns the amalgamation of organized crime and terrorism into a single phenomenon. This might be illustrated by, for example, terrorist groups engaging in bank robberies and other criminal activities to finance their operations, and terrorist groups becoming closely linked to, and in some cases actually involved in, drug trafficking.
The second model is the "organized crime-terrorism nexus" thesis, which concerns the cooperative collusion between organized crime and terrorism. Williams suggested that this might be illustrated by arms for drug deals and the payment of tribute or taxes to terrorist groups by organized criminal groups for moving contraband through their territory.
Williams' third type of interaction is "transformation", which concerns the mutation or migration of criminal activities to political terrorism or vice versa. This captures situations where a terrorist group reduces its terrorist activities and increases criminal practices; or a criminal organization becomes highly politicized and radically alters the focus of its activities. For instance, this last instance could be observed in the behaviour of some organized criminal groups who actively target and kill political candidates and members of local and national governments.
Makarenko has identified four types of relationships across a "crime-terror continuum" (Makarenko, 2004). According to Makarenko, a continuum "illustrates the fact that a single group can slide up and down the scale - between what is traditionally referred to as organized crime and terrorism - depending on the environment in which it operates" (Makarenko, 2004). The first type of relationship, alliance, occurs when criminal groups form alliances with terrorist organizations and terrorist groups seek alliances with criminal organizations. These relationships can be a one-time occurrence, or they might last for a short or long period of time depending on the reasons for the alliance, such as exchange of expert knowledge (money-laundering, bomb-making, etc.) or operational support (access to smuggling routes, transport of smuggled drugs/arms/people, etc.).
The second relationship Makarenko refers to as operational motivations, which are the efforts on the part of criminal and terrorist groups to bring each other's tactics in-house in order to enhance security and operations. Makarenko posits that criminal use of terrorist tactics can be traced back through the history of organized crime, yet terrorist use of criminal activities is a fairly recent development. Increasingly, criminal groups engage in political activity to alter operational conditions, especially in fragile States; and simultaneously, terrorist groups escalate their focus on criminal activities as a way to compensate for lost financial support from State sponsors. The third relationship, convergence, entails the transformation and mixing of tactics and motivations so much so that "criminal and terrorist organizations could converge into a single entity that initially displays characteristics of both groups simultaneously; but has the potential to transform itself into an entity situated at the opposite end of the continuum from which it began" (Makarenko, 2004).
Finally, Makarenko defines the fourth type of relationship across the crime-terror continuum as the black hole, which are "situations in which weak or failed states foster the convergence between transnational organized crime and terrorism, and ultimately create a safe haven for the continued operations of convergent groups" (Makarenko, 2004). This "most extreme" relationship on the continuum occurs when groups engaged in civil war change motivation from political to criminal, as well as when a State is effectively taken over by a hybrid group (a group whose political and criminal activities are deeply intertwined) and subsequently becomes a 'black hole' State.
The relationships articulated along Makarenko's crime-terror continuum reveal the flux nature of criminal and terrorist groups and their activities, as well as highlight the fickle nature of interaction between transnational organized criminal groups and terrorist groups which makes it especially difficult to develop law enforcement and counter-terrorism responses to confronting the crime-terror nexus.
Shelley et al. provide another model to examine crime-terrorism linkages (the terror-crime interaction spectrum), and identify five types of interactions: activity appropriation, nexus, symbiotic relationship, hybrid, and transformation. Similar to Makarenko, Shelley et al. contend that movement across the spectrum is possible; groups might move forward, backwards, skip steps, or maintain a consistent form of interaction (Shelley et al, 2005). Activity appropriation occurs when terrorist organizations and organized criminal groups adopt the other's methods without actually collaborating or working together. This interaction does not represent a firm linkage between the groups but does highlight how methods can be employed across the groups with relative ease. In some cases, the groups may proceed to form a nexus interaction, where terrorist groups come into regular contact with organized criminal groups to fill a need, such as acquire forged documents or launder money. A terrorist group may not engage in a nexus interaction because it cannot find a crime group willing to do business with it or because the terrorist group sees no benefit in working with people outside their group. In this case, the terrorist group may engage in transformation and fully engage in criminal activities; profits eventually overshadow the old objectives and terrorist methods. The nexus interaction is instrumental in developing a symbiotic relationship between organized criminal groups and terrorism groups. In fact, Shelley et al. contend that a long-term nexus interaction will ultimately result in deeper relations to form a symbiotic relationship interaction: a relationship of mutual benefit or dependence. Finally, hybrid interaction results when "two groups continue to cooperate over a long period and members of the organized crime group begin to share the ideological goals of the terrorists [...] grow(ing) increasingly alike and finally they merge." A hybrid organization engages in criminal activities and also has a political agenda. Shelley et al. note that once two groups reach the point of hybrid interaction, there is no reason to assume transformation will then occur. Additionally, appropriation of activities does not in any way imply future cooperation between groups. Noting this ever-changing landscape, the Shelley et al. study focuses on identifying and examining the political, economic, and social landscape for specific crime-terrorism interactions; with the goal of establishing "watch points" where interactions may occur.