Russia has made several arrests, also discovering and disarming an explosive device in an apartment where suspected accomplices of the St Petersburg attacker lived. Investigators said the raids also provided other leads.
Russian authorities on Thursday detained three people suspected of being accomplices in Monday's metro bombing in St Petersburg. The attack killed 14 and injured 50 more.
The flat was raided at around 5 a.m. local time (01:00 UTC) in Tovarichesky Prospekt, east of St Petersburg.
Tenants living in flats on two stairwells were also told to leave the building after authorities discovered an explosive device which was subsequently made safe.
Russia's Investigative Committee - the country's top criminal investigation authority - said that the suspects arrested on Thursday hailed from Kyrgyzstan, like the presumed attacker.
Investigation into radicalization
Akbarzhon Djalilov, a Russian citizen born in the Central Asian nation of Kyrgyzstan, is currently the prime suspect in Monday's underground bombing.
The 22-year-old, who authorities believe was radicalized by Islamic extremists, was killed in the explosion.
According to the Investigative Committee, Djalilov also left a second bomb at a different metro station last Monday. That bomb, however, failed to detonate.
Another group of six Central Asians was detained in a raid in St Petersburg earlier this week. The group is believed to have acted as recruiters for terrorist groups, including the so-called 'Islamic State' (IS). No direct link was discovered between that group and Djalilov .
ksb/msh (Reuters, dpa)