Six people have been arrested in St Petersburg on suspicion of recruiting for Isis and involvement in terrorist activity, investigators have said.
It said they were suspected of recruiting other migrants from Central Asia "to carry out terror-related crimes" and join extremist groups that are banned in Russian Federation, including Islamic State (IS) and the Al-Nusra Front.
"Searches are being held at the suspects' living places, in the course of which investigators are seizing Islamist extremist literature, the items and documents important for investigating the criminal case", the Investigative Committee said.
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.
"As for Akbarzhon Dzhalilov, who, according to the investigation data, set off an explosive device in the train's auto, his identity has been confirmed by the results of a DNA test", the committee's spokeswoman Svetlata Petrenko said.
The predominantly Muslim Central Asian states are seen as a prime recruiting ground for Islamic militants.
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"The recent tragic events in St Petersburg are the best confirmation of this".
Investigators scoured the St. Petersburg home of Dzhalilov and reviewed surveillance footage that shows him leaving his home with a bag and a backpack.
"We know that each of our countries, practically every one, is a possible and potential target of terrorist attacks".
The remains of Akbarzhon Dzhalilov, 22, were recovered from the scene of the bombing, Russian authorities said on Tuesday.
The bomb went off on a train under Russia's second-largest city on Monday.
On Monday, an explosion occurred in the St. Petersburg underground on the stretch of rail between the Sennaya Ploshchad and Tekhnologichesky Institut metro stations at around 3 p.m. local time (12:00 GMT). Authorities refused to confirm that the couple were Jalilov's parents but the woman, in response to a Russian TV reporter's question, said she did not believe her son was the bomber.