By Stephen Mulvey
Islamist radicals from the North Caucasus are nearly always at the top of the list of suspects when a bomb causes death and destruction in Russia.
Female suicide bombers killed 15 at an open-air rock festival in July 2003
All the more so, when the attack is carried out by female suicide bombers.
Guerrillas fighting to separate the republic of Chechnya from Russia adopted the suicide bombing tactic for the first time in 2000, but initially the bombers were men.
Female suicide bombers emerged in 2003 - though women wearing veils and bandanas indicating their readiness to die in battle had made a first appearance a year earlier, during the siege of a Moscow theatre.
They died along with the other hostage takers and 129 members of the audience, when the building was gassed and stormed by security forces.
One of the first cases of female suicide bombing was an attempt to kill the pro-Moscow Chechen leader, Akhmad Kadyrov (father of the republic's current president), at a religious ceremony in May 2003.
In July and December of the same year, they struck in Moscow itself, on the first occasion killing 15 at an open-air rock festival.
Six people died in a Moscow bombing blamed on women in December 2003
Women were also part of the gang that took over a school in Beslan, North Ossetia, in 2004 - an attack which ended in 334 deaths, including 186 children.
It's therefore no surprise that the head of the Russian Federal Security Service, Alexander Bortnikov, has suggested that the suicide bombers who struck the Moscow Metro came from the North Caucasus.
The question of why these women have been prepared to take their own lives in order to kill others has been given a partial explanation.
Two of the women who joined the Moscow theatre siege are reported to have been sisters seized from their homes in a Chechen village by Russian soldiers and gang-raped.
Over the past 14 years, Russian soldiers have left a trail of destruction in Chechnya that is psychological as well as physical.
Countless women have been widowed, or lost sons, brothers or fathers. Those who have been raped may find it impossible to marry and live a normal life.
Post-traumatic stress disorder is reported to be widespread among Chechen women.
One of the most notorious Chechen rebel leaders, Shamil Basayev, who was killed in 2006, boasted of having a battalion of female suicide bombers.
At least 35 people have been killed in the latest suicide bombing
For a few years after Beslan, no suicide bombings were carried out - but that changed in 2009.
The current Chechen rebel leader, Doku Umarov, has been declared "emir" of the North Caucasus, a patchwork of mountain republics inhabited mostly by Muslims.
He has announced his intention to install Sharia law across the region, and described the Chechen struggle as part of a wider one between Muslims and the West.
It's unlikely he would hesitate to use female suicide bombers, who Moscow police may have regarded as less suspicious than North Caucasian men - for the last few years at least.
This latest attack, it is widely thought, could be retaliation for a raid last month by Russian forces in Ingushetia, Chechnya's western neighbour, in which 20 insurgents died, including the presumed leader of a bomb attack on a Moscow to St Petersburg express train last November.