Six individuals have been arrested in St. Petersburg on suspicion of abetting terrorism, RAPSI learnt from the Investigative Committee on Wednesday.
Those detained had been recruiting other Central Asian immigrants in St Petersburg "to carry out terrorist crimes" and to join "illegal armed groups", including IS, Russia's state investigative committee said in a statement.
The committee said that their homes were being searched and extremist literature had been confiscated.
The committee said now there is no evidence that the six detainees have links with the perpetrator of the deadly blast, but it will check all their contacts and connections.
On Wednesday, Russian President Vladimir Putin acknowledged the rising terrorist threat, affecting not only Russia, but also its allies in the Commonwealth of Independent States (CIS) - an organization embracing 11 former Soviet countries.
"The recent tragic events in St Petersburg are the best confirmation of this".
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Mr Putin said: "We see that, unfortunately, the situation is not improving".
"As for Akbarzhon Dzhalilov, who, according to investigators, had detonated the bomb inside the train carriage, his identity was confirmed with DNA tests", Petrenko said.
"We know that each of our countries, practically every one, is a possible and potential target of terrorist attacks", he said.
It said surveillance cameras had captured the moment when he left his building carrying a bag and a knapsack.
Parts of the bomber's body were found in the wreckage of the metro carriage after the attack, which occurred between Sennaya Square and Technology Institute stations, the Russian authorities said.
He was an ethnic Uzbek and naturalised Russian citizen, who had remained in St Petersburg to work in a sushi restaurant after his parents returned to Kyrgyzstan, according to a spokesman for the Kyrgyz government's Committee for National Security.