When ISIS took over Iraq’s second city of Mosul, Western intelligence agencies were concerned that the terrorist group could potentially get its hands on radioactive sources located on a college campus, which would be sufficient to build a “dirty bomb.” However, as it turned out after the liberation of Mosul from ISIS, the terrorists never touched the cobalt-60 machines.

From the early 1990s onwards, many terrorism experts were expressing the idea that there had been a transformational shift in the way that terrorism was being conducted.  After the end of the Cold War, it was perceived that terrorist organizations were no longer interested in their traditional political objectives of national liberation or Marxist-Leninist ideologies, but were instead primarily motivated by religious fanaticism or identitarian concerns. Rather than utilizing terror and destruction to achieve a political goal, the act of inflicting maximum violence had become the objective itself. “New terrorists” no longer had to illicit sympathy or appeal to a broad constituency but instead only sought to punish and destroy their perceived enemies.

The terrorism analyst Brian Jenkins later summarized this trend when he stated that while previously “terrorists wanted a lot of people watching, not a lot of people dead,” they now wanted “a lot of people watching and a lot of people dead.”  At the same time there was the view that the collapse of the Soviet Union, globalization and improvements in telecommunications had made CBRN (chemical, biological, radiological, nuclear) material and expertise more readily available than ever before. The combination of these two factors meant that mass casualty attacks, particularly those involving unconventional weapons, were perceived as both inevitable and imminent. The transnational nature of some “new terrorist” organizations combined with their independence from the restraints imposed by state sponsorship made them apparently well suited to exploit this new environment. This change in nature was then seemingly proven by the 1995 Tokyo subway Sarin attack by Aum Shinrikyo and the events of 9/11.

While the concept of “new terrorism” is still being debated, there appears to have been an increasing trend among contemporary terrorist groups to explore CBRN attack options. The clearest reason that certain terrorist organizations may be drawn to CBRN materials is their potential destructive capability: an improvised nuclear device, if detonated successfully in a metropolitan area, would likely cause unprecedented numbers of casualties. The asymmetric potential for a small terrorist cell operating without restraint to inflict so much destruction gives CBRN devices obvious appeal. Furthermore, the apocalyptic imagery associated with CBRN materials may appeal to death cult-like groups. For instance, Aum Shinrikyo predicted the end of the world and thus investigated a range of chemical, biological and nuclear attack options to bring that to pass.

The unnerving nature of CBRN weapons may also provide a means by which to cause “extra terror”. Evidence to this end is provided by chlorine bombings carried out by the Islamic State of Iraq (ISI) in 2007, and many chemical attacks years later by ISIS in Iraq and Syria. This has been interpreted as a further attempt, in conjunction with conventional terrorism, to dissuade the local population from cooperating with the Iraqi government. The novelty and taboo surrounding CBRN weapons may additionally serve as a means by which to conduct spectacular attacks, especially when conventional mass-casualty terrorism has become almost “the norm”. Al-Qaeda and ISIS in particular seem to have been interested in acquiring a sophisticated CBRN device to demonstrate their technical capability.

While there have been a number of dire predictions that suggest mass-casualty CBRN terrorism is highly likely and probably imminent, these assessments often fail to take account of the significant barriers that would be encountered by a terrorist organization. Even before an attack can be conducted, the task of obtaining or creating a sophisticated CBRN weapon poses an immense challenge. The logistical, financial and technical hurdles involved mean that advanced CBRN capabilities will only be within reach of the most organized and possibly state-supported terrorist groups such as Lebanese Hezbollah. Aum Shinrikyo were unable to buy a significant CBRN capability on the black market, even with a practically unlimited budget. Instead, they were forced to painstakingly develop a chemical weapons capability with a team of their own scientists over a period of years.

When the difficulty of terrorists developing a major CBRN weapon is combined with the limited results in scale demonstrated by their use thus far, it becomes apparent why so relatively few groups attempt such a project. While CBRN weapons theoretically have immense destructive capability, a terrorist-made CBRN weapon would most likely be created in a clandestine environment, under time and expertise constraints and with whatever material was readily available. Testing may prove impossible—among other reasons for fear of detection—and there may be a tactical imperative for the weapon’s immediate use. It is therefore rather unsurprising that terrorists have mostly been limited to a basic CBRN capability. For example, Aum Shinrikyo’s diluted chemical agent and inefficient dispersal technique meant that their gas attack killed comparatively few, particularly when compared to the tens of millions of dollars that was spent on their unconventional weapons program.

In comparison, conventional terrorist attacks such as those in Paris and Mumbai prove that CBRN materials are not necessary to cause mass casualties. Therefore, in light of the greater operational challenges involved in using CBRN materials, it is logical in a cost-benefit sense why conventional attacks are more prevalent, even amongst groups seeking to cause high numbers of casualties. Finally, it is worth noting that even a group with seemingly maximalist or apocalyptic goals may decline to use CBRN materials. Some have argued that even the most extreme terrorist organizations could be considered rational to the extent that they would collectively seek to continue their group’s existence. The potential retaliation to a major CBRN terrorist attack may therefore be too great for it to be a valid strategy.