London terrorist attack highlights vehicle-impact as most likely lone-actor tactic for mass-casualty attacks in UK

On 22 March at approximately 1435 local time, a vehicle drove the length of London's Westminster Bridge, mounting the pavement and hitting pedestrians. After hitting the perimeter fence of Westminster Palace, the assailant gained access to parliament grounds and fatally stabbed a police officer, before being shot dead by police officers. According to BBC News on 23 March, at least three people were killed by the attacker and at least 29 wounded, with seven sustaining critical injuries. The attacker's identity has yet to be revealed by police. The incident represents the most severe attack in the United Kingdom since the 7 July 2005 Al-Qaeda suicide attacks in London, when four attackers detonated improvised explosive devices (IEDs) on the transport network, killing 52 people and wounding several hundred.

Expected tactic

The attack – which is being treated as Islamist terrorism by the Metropolitan Police – is in line with IHS Markit's terrorism risk assessment that attacks in the UK are most likely to be conducted by lone actors, with attacks using vehicles and knives highlighted as a predominant tactic. It also followed several mass-casualty vehicle-impact attacks conducted in Europe in 2016, which signals an increasing use of the tactic by Islamist extremists. In Nice, France, on 14 July 2016, Mohamed Lahouaiej-Bouhlel drove a truck into pedestrians on a promenade killing 86 people and wounding more than 300, and in Berlin, Germany, on 19 December 2016, Anis Amri drove a truck into a Christmas market, killing 12 people and wounding 56. Although no group has yet claimed responsibility for the attack, it would not be unusual for the Islamic State to claim the attack without being intricately involved in its execution; the group has frequently called for such attacks to be conducted in the West during the past year.

Outlook and implications

A police officer stands guard on 23 March, the day after a terrorist attack in Westminster, London.

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Potential future terrorist attacks in London and the UK more broadly are most likely to follow a similar low-capability/high-impact pattern, with less likelihood of more complex and drawn-out attacks being conducted by organised cells of multiple people. Strict gun-control laws in the UK and the difficulty of acquiring automatic weapons, even on the black market, decrease the likelihood of mass-casualty shooting attacks. The risk that vehicular attacks will therefore be used as the most likely tactic of lone actors intent on conducting mass-casualty attacks in the UK is elevated.

Crowded tourist attractions – particularly with wide open spaces and adjacent to high-profile locations – remain the most probable to be targeted in the capital, as they are difficult to secure against such attacks. The attack and its prominent location has the potential to also negatively impact the tourism sector over the coming summer months, with London now demonstrably not immune from the types of attacks seen in other European countries in 2016.

Although there is no indication that the attack was conducted by more than one person, police have since raided a number of properties in London, Birmingham, and elsewhere in the UK, arresting six, reportedly in connection with the 22 March attack. Further raids are likely over the coming days, increasing risks of disruption to business, while the area around Westminster will remain cordoned off. Meanwhile, the Metropolitan Police announced on 22 March that security will be increased across the London area in the coming days and weeks to prevent further attacks, with more officers deployed across the capital. The attack is also likely – particularly as police said they assumed the attack was inspired by "international terrorism" and "Islamist-related terrorism" – to prompt an increase in small-scale violence by right-wing extremists targeting Muslims and ethnic minorities.