Failed detonation in London underground attack indicates that militant IED attack intent often exceeds capability

Members of the emergency services work outside Parsons Green underground tube station in West London following a terrorist incident on 15 September 2017.

Daniel Leal-Olivas/AFP/Getty Images

According to police, most of the injuries from the attack were "flash burns", although some were caused in the subsequent panic when passengers fled the scene. The improvised explosive device (IED), a wired plastic bucket inside a plastic cooler bag, was attached to a timer and the explosive failed to detonate as planned. Police are treating the attack as a "terrorist incident", and the attack was claimed by the Islamic State via its Amaq News Agency later on the same day.

The United Kingdom's terror threat level was raised to 'Critical' in the immediate aftermath of the attack, indicating that another attack was "expected imminently". The decision is likely to have been motivated by the fact that the perpetrators had not been immediately identified and remained at large. Furthermore, the timer on the IED may well have caused security services to assess that the attack at Parsons Green was not intended as a suicide mission, raising the risk of a further attack. Troops were deployed to reinforce police, predominantly in a static protection role, guarding sensitive sites. Police have since arrested two men, aged 18 and 21 respectively, after which the terror threat level was reduced to 'Severe', implying that there was no further indication of an imminent further attack.

Use of explosives

The IED contained triacetone triperoxide (TATP), the same explosive used in the 2005 attacks on London's transport network and the Manchester Arena attack on 22 May 2017, in which 56 and 23 people were killed respectively. The same explosive was also used in the November 2015 co-ordinated Paris attacks, the March 2016 Brussels attacks, and the Catalonia attacks in August, firmly placing it as the jihadist explosive of choice in Western Europe. One of the likely main reasons is that, unlike other crucial ingredients for IEDs, which are tightly controlled in the European Union, chemicals such as peroxide and acetone used in TATP are widely available in household products. Purer forms of these precursor chemicals can also be bought on the internet, and instructions on the manufacture of IEDs using TATP are available on jihadist websites.

Nevertheless, the malfunctioning of the device deployed in Parsons Green reflects the difficulty of constructing viable, high-capacity IEDs, particularly with highly volatile TATP, without a high degree of expertise. Furthermore, the use of IEDs in an attack also raises the risk of detection by security services, due to the need to acquire all the necessary components, construct, and deploy the IED, without detection. Although IEDs have been frequently used in attacks in Europe, since 2014 only five IED attack attempts attributed by Jane's Terrorism and Insurgency Centre to Sunni Islamist militants have resulted in casualties. This indicates that the militants' strong intent to develop and employ high-yield explosives often exceeds their capability.

Outlook and implications

The Parsons Green attack is the fifth terrorist attack (the fourth with a jihadist motive) to target the UK in 2017, highlighting a step-change in momentum. It underlines the UK's high-risk exposure to both low-capability and more sophisticated IED attacks carried out by lone actors or, more often, small cells inspired but not necessarily directed or assisted by non-state armed groups, such as the Islamic State. Although the use of rudimentary weapons such as knives and vehicles will remain a highly likely method of attack, recent incidents underline the capacity of militants to construct and deploy IEDs. Cells in which the membership includes at least one returnee foreign fighter, trained in the construction of explosives, are far more likely to succeed in deploying viable devices. Although London's Metropolitan Police have not provided many details yet on the arrested suspects, or potential links to a wider support network, failed or disrupted attacks have previously served as catalysts, accelerating pre-existing plots or prompting militants, usually part of the same network, to conduct a less sophisticated impromptu attack. This was evident in the vehicle-impact and knife attacks in Barcelona and Cambrils, Spain, on 17 August, which were likely triggered by the accidental destruction of an Islamist militant network's bomb-making facility in nearby Alcanar. The 2016 Brussels attacks also followed a similar pattern.