"Investigators are establishing whether or not he travelled to Syria to take part in hostilities".
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. The identification of Kyrgyz-born Akbarzhon Jalilov as the bombing suspect stoked fears of police action targeting migrants from Central Asia and the Caucasus. Fifty-five people wounded in the blast remain hospitalized, deputy mayor Anna Mityanina said Wednesday.
Searches are being conducted at the suspects' places of residence; law enforcement officers are seizing extremist literature, document and other items significant to the case, investigators said.
Meeting with the heads of security services from a regional alliance that includes most of Russia's Central Asian neighbors, President Vladimir Putin warned that terrorism remained a threat to all in the region.
There are hundreds of thousands of central Asian emigres living and working in Russian Federation.More news: Trump signs repeal of US broadband privacy rules
Dzhalilov carried a bomb in a backpack that exploded on the train as it was traveling in between two stations, investigators said.
Mr Putin said: "We see that, unfortunately, the situation is not improving".
It said surveillance cameras had captured the moment when he left his building carrying a bag and a knapsack.
"We do know that each of our countries, nearly every country, is a potential target of a terrorist attack", he said. 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.
"The recent tragic events in St Petersburg are the best confirmation of this". But parliament speaker Vyacheslav Volodin dismissed the statements as attention-getting devices and upbraided the politicians, saying "One must not use a tragedy to promote oneself".