If Russia's security services are correct in blaming two Chechen "black widows," or female suicide bombers, for attacks that killed almost 40 people on Moscow's crowded metro Monday morning it represents the return of a nightmare that Kremlin thought it had ended years ago.
The use of women to strike civilian targets was pioneered a decade ago by Chechen warlord Shamil Basayev, who was seeking a way to smuggle explosives past Russian checkpoints at the outset of the Kremlin's second war to subdue separatist Chechnya, say security experts.
"Basayev started up a 'martyr's brigade' comprised of women, who proved very successful in killing Russian officials and destroying administration offices," says Andrei Soldatov, editor of Agentura.ru, an on-line journal that focuses on Russia's security services.
"After Basayev was killed by Russian forces in Ingushetia in 2006 we were told that the martyr's brigade had been disbanded," says Mr. Soldatov. "Some rebel websites have since claimed that it was resurrected, but only now do we see clear evidence that it's back."
The women, who call themselves "shakhidy," or martyrs, are typically the widows or mothers of Chechen men who've been killed by Russian forces.
"Chechen society is very hard for women who've lost their menfolk, or who have no breadwinner, and they become vulnerable to recruitment," says Soldatov. "There are potentially very many such women in the north Caucasus at the present time."
Russian security forces call them "black widows," both for their deadly intentions and the head-to-toe black mourning clothes that are their signature dress. A large contingent of female shakhidy were among the 50 Chechen rebels who seized a central Moscow theater in 2002, and subsequently died along with 120 hostages when Russian security forces stormed the building using a poisonous knock-out gas.
Two black widows blew themselves up at a Moscow rock concert the next summer, killing 14 people.
Another pair have been blamed for destroying two Russian airliners in 2004, just days before a large group of militants -- including several black widows -- seized a school in Beslan, North Ossetia, with 1,200 hostages. More than 300 people, half of them children, died in the bloodbath that ensued when Russian forces stormed the school.
Experts say that increasing numbers of family members, including women, are turning to radicalism as conditions in the north Caucasus spiral out of control and Moscow-backed security forces crack down hard on insurgents, often punishing families and even whole villages suspected of aiding them.
"When an elder brother is kidnapped or a relative is killed, the younger brother takes arms," says Tatiana Kasatkina, executive director of Memorial, Russia's largest independent human rights monitoring group.
"Thus the soil is created," by arbitrary and often brutal actions of the authorities, she says. "Young people have no patience and they often do not see any other means to fend for themselves and their family but to take up arms."