Moscow said that there were an additional twenty-one " black widows " ready to strike , and were studying if the alleged attackers were part of an original group of thirty . Female suicide bombers , known as " black widows , " have been involved in suicide bombings on numerous occasions , including 2004 bombings of two passenger planes that took off from Domodedovo International Airport , previous Moscow metro bombings , the Moscow theater hostage crisis , and the Beslan school hostage crisis
Moscow theater hostage crisis
Moscow theater hostage crisis
2010 Moscow Metro bombings