Blind Spot Did Russian Intelligence Neglect The Islamist Threat?

As Russia observed a day of mourning last Sunday for the victims of the terror attack two days earlier, along with the sorrow came the hard question that follows most similar incidents: how could this have happened?

- Shaun Walker, Pjotr Sauer, Andrew Roth

Rooting out determined and well-trained terrorist cells is not an easy task for security services in any country, but there are numerous signs that failing to prevent last Friday's attack was in large part down to a catastrophic security failure on the part of Russian authorities.

First, there was the public warning from the US government earlier this month that it had learned of "imminent plans to target large gatherings in Moscow" by terrorists.

The warning, also shared privately with the Russian government, suggested Washington had picked up some fairly specific intelligence relating to an upcoming attack. But Vladimir Putin, three days before the attack, brushed off these warnings, calling them an "attempt to scare and intimidate our society".