A suicide bomber was responsible for the deadly blast in the St Petersburg metro on Monday, a law enforcement source told Interfax, citing preliminary information.
The source also told Interfax that authorities had established the identity of the suspected suicide bomber and that the suspect was a 23-year-old from central Asia who had carried an explosive device into the St Petersburg metro in a rucksack.
A man who was captured on surveillance cameras and earlier suspected of a role in the blast came forward to police and said that he played no role, Interfax reported.
Separately, Russian news agencies reported that Russian President Vladimir Putin had met with security services over the incident and laid flowers at the metro station where the blast happened.
Russia's REN-TV published an image earlier on Monday it claimed showed the man who allegedly planted a bomb in a carriage in St. Petersburg's metro.
Russia's state investigative committee said it had opened a criminal case over the bombing on charges of a terrorist act. The agency, which has sweeping powers, said it had sent a group of investigators to St. Petersburg, adding it would consider other possible causes of the incident besides terrorism.
Interfax quoted a security source as saying that Russia issued search warrants for two people for their possible role in the blast. One of the suspects "is thought to have placed the explosive device in the metro wagon and the second person [is sought] for leaving a bomb at the metro station 'Ploshchad Revolutsii'," the source told Interfax.
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