Moscow is holding an official day of mourning for the 39 people killed in Monday's twin suicide bombings on the city's Metro.
Russian President Dmitry Medvedev laid a wreath at Lubyanka metro station. He called the plotters "beasts", adding: "We will find and destroy them all."
Officials have blamed Muslim groups from the northern Caucasus, where rebels have been waging violent campaigns for independence.
Dimitri Babich, analyst with Russia Profile, explains the background to the conflict in the northern Caucasus.