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Authorities in Russia have initially identified the suspect behind the St Petersburg metro blast.
They believe a young person, believed to a 23-year-old man from Central Asia and linked with radical Islamists, was a suicide bomber and carried an explosive device onto the train in a rucksack.
Police believe the same man, not a second suspect, planted a bomb at another subway station that was defused before it went off.
Floral tributes and candles have been left by mourners in memory of the victims of the metro bombing in St Petersburg.
The tributes were left at an improvised memorial on Moskovsky Prospekt Street near the entrance to Tekhnologichesky Institute station.
The death toll in the subway bombing in St. Petersburg has reached 11, Russia's top anti-terror agency has said.
Russia Prime Minister Dmitry Medvedev has described the blast on the St Petersburg metro station as a "terrorist attack".
In a post on his twitter account, Mr Medvedev sent his "most sincere condolences to the families and relatives of the victims of the explosion" and said the injured would be given "all necessary assistance".
Search warrants have been issued for two people over the St Petersburg metro blast, according to Interfax news agency.
Officials in Paris have increased security at public transport locations "as a precautionary measure" following the explosion in a St Petersburg underground tunnel on Monday.
In a statement, the interior ministry said: "Following the events in the St. Petersburg metro, and as a precautionary measure, Interior Minister Matthias Fekl has decided to redeploy security means in public transport across the Paris region.
"Amid an extremely high terrorism threat, the government continues to take measures to protect the French people."
At least nine people are believed to have been killed and 20 injured after an explosion in St Petersburg, according to Russia security officials.
Russia's National Anti-Terrorist Committee also confirmed the blast occurred between the Sennaya Ploshchad and Tekhnologichesky Institut metro stations in the city.
Latest ITV News reports
The seven people were from Central Asian countries and investigators say they were recruiting for IS.
The bombing of a subway train in Russia has drawn condemnation from world leaders.