Terrorism & Insurgency

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

19 September 2017
Members of the emergency services work outside Parsons Green underground tube station in West London following a terrorist incident on 15 September 2017. Source: Daniel Leal-Olivas/AFP/Getty Images

Key Points

  • The United Kingdom's terror threat level being reverted to 'Severe' from 'Critical' implies that there is no further indication of an imminent further attack.
  • The malfunctioning of the device reflects the difficulty of constructing viable, high-capacity improvised explosive devices, particularly with highly volatile triacetone triperoxide (TATP).
  • The return of foreign fighters with experience in constructing explosives abroad would increase local jihadist networks' capability to deploy viable devices.


On 15 September, an improvised explosive device partially detonated on a London underground train carriage during the morning rush hour at Parsons Green station, West London, wounding 30 people.

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.

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