‘Four Lions factor’: how terrorist incompetence is saving lives

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.