Eight people have been arrested in connection with Monday's deadly bomb attack in St. Petersburg, the Russian Investigative Committee said in a statement.
Fourteen people were killed and dozens more injured Monday when a suicide bomber detonated explosives on the city's metro system.
On Thursday, investigators arrested six people in St. Petersburg and two others in Moscow, the committee said in a statement.
Three of those arrested in St. Petersburg were Central Asian men who had contact with the bomber, Akbarjon Djalilov, Vladimir Gaidei, deputy administrator of the city's Nevsky district, told CNN.
The DNA of Djalilov, 22, a Russian national born in the central Asian Republic of Kyrgyzstan, was matched to a bomb left at a second metro station on Monday that was defused by authorities, according to Russian officials.
During Thursday's arrests, an identical bomb was found at a residential address, the committee said.