Not for the first time it seems that a terror attack - this time in Parsons Green - could have been much worse but for a malfunctioning device

Witness accounts of the “fireball” on a tube train at Parsons Green in London and the images of the bomb apparently responsible suggest an improvised explosive device that did not function as intended.

Metropolitan police sources said the device only partially exploded, and initial examination of the device led explosives experts to conclude it was “viable”, meaning it was meant to explode more fully.



One possibility is that the detonator did not set off the main charge, causing the flash and the heat that burned some commuters but not a powerful blast that could have been much more destructive.

This has happened before: on 21 July 2005, when a series of bombs hidden in backpacks and placed on tube trains by a network of Islamic militants failed to go off.

That attack came just two weeks after 52 people were killed in the 7/7 multiple suicide bombings in London targeting public transport. Police rapidly traced those responsible for the 21 July abortive attempt and detained them. Analysis found the homemade explosive they had manufactured had not ignited.

Play Video 1:31 'A fireball singed my hair': what Parsons Green witnesses saw – video report

The incompetence of terrorists has spared hundreds of lives in recent years. The recent attacks in Barcelona could have been much worse if the leader of the plot had not blown himself up – along with the network’s stockpile of bomb components – hours before they occurred.

Among the many failed incidents in the UK are attempts to bomb a cafe in Exeter (that failed when a bomber set off his own device in a toilet); to bomb a nightclub in London with incendiary devices (that smouldered but did not burn), and to bring down a transatlantic passenger jet (with a bomb in a shoe that proved impossible to ignite).

In the US, a massive blast was avoided in Times Square, New York, because the bomber programmed the wrong time, while in Yemen in 2000 an attempt to sink a US navy ship failed when a dinghy overloaded with explosives sank when it was launched.

The same goes for attacks by extremists motivated by other ideologies. Well under a half of the 150 far-right plots recorded by the Anti-Defamation League in the US between 1993 and 2016 succeeded. In Columbus, Ohio, in April 2016, a rightwing extremist blew off his own hands while allegedly making an explosive that authorities said was to be used as a diversion during a bank robbery.

In an incident in Spokane, Washington, in 2011, the FBI arrested a white supremacist who planted a pipe bomb on the route of a Martin Luther King Day parade. It failed to detonate.

Counter-terrorist specialists in the west recognise that the “Four Lions factor” – a reference to the 2010 black comedy by Chris Morris that shows the incompetent attempt by a group of Britons to launch a terrorist campaign – is one of the most important defences against attack.

Putting pressure on safe havens overseas to limit the ability of terrorist groups to provide training, stopping militants from travelling to those that do still exist, increasing the pressure on local networks and limiting communication with expert handlers, while of course making it harder to obtain crucial ingredients for bombs, all help ensure potential attackers remain without the means to realise their destructive ambitions.

So too does the elimination of key individuals with high levels of expertise. Western and Middle Eastern intelligence agencies have been trying for years to kill Ibrahim al-Asiri, an al-Qaida extremist in Yemen responsible for a series of ingenious devices that have repeatedly come close to causing appalling destruction. One device would have brought down a passenger plane over the US in 2009 if the bomber had been able to ignite it.

Asiri is at large, despite the decade-long effort to kill or capture him. And so is whoever was responsible for the device left on the tube train in London on Friday.