The unusual step follows a double suicide bombing on the Moscow metro last month that left forty people dead. The attack renewed fears of a fresh terror campaign in Russia's big cities and triggered a massive security crackdown. Interest in "black widow" suicide bombers has been intense ever since it was confirmed that the two women who struck the Moscow metro were indeed the the wives of deceased Islamist terrorists. Both came from the southern largely Muslim Russian republic of Dagestan in the North Caucasus.