A blast in a St Petersburg train carriage on Monday that killed 11 people and injured 45 was carried out by a suspected suicide bomber with ties to radical Islamists, Russia's Interfax news agency cited a law enforcement source as saying.
Russian President Vladimir Putin, who was in the city when the blast struck, visited the scene of the explosion late on Monday night and laid a bunch of red flowers at a makeshift shrine to the victims.
Witnesses said they saw passengers who were bloodied and burned spilling out of the train, whose door was buckled by the force of the explosion, and lying on a platform while smoke filled the station.
Russia has in the past experienced bomb attacks carried out by Islamist rebels from Russia's North Caucasus region. The rebellion there has been largely crushed, but Russia's military intervention in Syria has now made it a potential target for Islamic State attacks, security experts say.
There was no immediate claim of responsibility. Officials said they were treating the blast as an act of terrorism, but there was no official confirmation of any link to Islamist radicals.
Earlier, Russian media had broadcast closed circuit television footage of a bearded man they said was being sought by police as a suspect. But Interfax reported that the man had come forward and been eliminated from inquiries.
The news agency, quoting an unnamed law enforcement source, said that human remains examined at the scene suggested that the blast had been carried out by a suicide bomber. It said the police had identified a suspect with links to radical Islamist groups banned in Russia.