Officials in Russian have identified a 17-year-old as one of the two female suicide bombers who struck in Moscow.
Dzhanet Abudrakhmanova is the widow of an Islamist militant leader killed in the volatile region of Dagestan late last year.
Her late husband, Umalat Magomedov, was shot dead by police on New Year’s Eve.
Chechen rebel leader Doku Umarov claimed in a video broadcast that he had ordered the two bombings to “destroy infidels” and seek revenge for the Kremlin’s policies in the Caucasus region.
Umarov warned that more attacks could follow.
Monday’s suicide bombings by the “Black Widows” killed 40 people and injured dozens more.
As Russia mourns the dead, authorities are stepping up security in public places.
Russian President Dmitry Medvedev has vowed to clamp down harshly on militant groups by whatever means necessary.