Russian authorities said on Tuesday a 22-year-old Akbarzhon Dzhalilov is the presumed suicide bomber who carried out the attack which killed 14 people and left 49 in hospital.
Six suspected ISIS and Al-Nusra Front recruiters, said to come from Central Asian countries, were detained in St. Petersburg days after the blast in the city's Metro.
"Russia's Investigative Committee will thoroughly check all the ties and the contacts of the persons detained but (right now) the investigation has no data on the detainees' link and acquaintance with the perpetrator of the St. Petersburg attack", it said.
The suspects are from Central Asian countries and the group of six men has been active since 2015, said theInvestigative Committee of the Russian Federation in a statement.
Police have also found items similar to fragments of the bomb defused at the Ploshchad Vosstaniya metro station during the search of Akbardjon Djalilov's apartment, Petrenko added.
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Investigators are now deciding on arresting the detainees and bringing charges against them.
"Unfortunately, there is no sign that the security environment is improving, and tragic events in St. Petersburg are the best evidence of it", Putin said while meeting chiefs of CIS intelligence agencies, as cited by TASS.
After the attack, several Russian politicians have called for ending the moratorium on capital punishment.
Investigators are now checking his contacts including in social networks where he could be recruited by some extremist and terrorist organisations.
"As for Akbarzhon Dzhalilov, who, according to the investigation data, set off an explosive device in the train's vehicle, his identity has been confirmed by the results of a DNA test", the committee's spokeswoman Svetlata Petrenko said. "However, he could be supervised via the Internet where he could receive instructions on how to make bombs", the source said.