What We’ve Learnt About Lone-Actor Terrorism Over the Years Could Help Us Prevent Future Attacks

It is very difficult – if not impossible – to determine the exact number of lone-actor terrorist attacks that have taken place in Europe with certainty. The absence of a universally accepted definition of terrorism is part of the problem. It’s also possible that acts of mass violence are being classified as terrorism when they are actually ideologically neutral. Equally, it can be difficult to determine whether an actor truly acted alone, especially in an age of internet radicalization.

What is clear is that independent terrorist attacks became more frequent in the early 2010s. By 2013, such incidents spiked, with Europe seeing six to seven Islamist and far-right attacks per year (up from fewer than one annually before 2010). These figures refer strictly to cases where perpetrators acted independently, excluding those with evidence of external support. For example, Anis Amri’s 2016 truck attack in Berlin and Taimour al-Abdaly’s 2010 suicide attempt in Stockholm were initially seen as lone-actor events, but later investigations revealed ties to Islamist cells.

Lone-actor terrorism appears less common among far-left and ethno-nationalist groups, though exceptions do exist.

Lone-Actor Terrorist Attacks in Europe
This shift towards lone-actor attacks is likely a result of evolving counterterrorism strategies implemented after major attacks like the 2004 Madrid train bombings and the 2005 London bombings. It became harder to carry out large-scale plots so groups like Al-Qaeda and later Islamic State switched to encouraging or organizing attacks by loosely affiliated individuals acting independently but on their behalf.

The struggle between terrorist groups and governments is one of constant adaptation. By 2018, Europol data indicated that all the Islamist attacks that had been seen through to completion in Europe during that year had been carried out by lone actors.

Lone-actor attacks have an even longer history within far-right terrorism. The term “lone-wolf terrorist” was first popularized in American white supremacist propaganda in the early 1990s – well before the more neutral term “lone-actor terrorist” was adopted by researchers. As counterterrorism efforts increasingly targeted white supremacist groups, many within the movement came to see independent action as the most effective way to evade detection and maintain operational secrecy.

Addressing the Threat
Fortunately, we now understand more about lone-actor crimes. We’ve come to understand that these attacks stem from complex psychological and environmental factors.

While perpetrators shouldn’t be dismissed as simply “crazy,” mental health can play a role in radicalization, especially when combined with personal grievances, failed aspirations, and perceived injustices. Influences from family, peers and online spaces also shape this process. While no two radicalization pathways are identical, certain patterns can be observed – and recognizing them early may help reduce the threat.

The idea of “self-radicalization” also merits caution. Lone actors rarely radicalize in isolation; their manifestos often echo broader ideological themes, shaped by conspiracy theories or charismatic figures. These actors often assign symbolic meaning to their actions. Raising awareness of the impact of violent public discourse is key – though this must be done without infringing on free speech. History shows that providing “pressure valves” for controversial ideas is more constructive than censorship.

Lone-actor attacks are, in part, difficult to prevent precisely because they are not a systemic threat in the way that coordinated, group-based terrorism can be. Its danger lies in isolated bursts of violence rather than in sustained campaigns. But there are patterns worth following that could help prevent future incidents.

Diego Muro is Senior Lecturer in International Relations, University of St Andrews. Ovidiu Craciunas is PhD candidate, The Handa Centre for the Study of Terrorism and Political Violence, University of St Andrews. This article is published courtesy of The Conversation.