A Russian suicide bomber originally from mainly Muslim Kyrgyzstan detonated the explosives in a St Petersburg train carriage that killed 14 people and wounded 50, authorities say.
The suspect had radical Islamist links, Russian media cited law enforcement officials as saying on Tuesday, raising the possibility Monday's attack could have been inspired by Islamic State, which has not struck a major city in Russia before. So far, no-one has claimed responsibility.
Kyrgyz officials identified the suspect as Akbarzhon Jalilov, born in the city of Osh in 1995, and Russian officials confirmed his identity, saying he had also left a bomb found at another metro station.
Biographical details pieced together from social media and Russian officials suggested Jalilov was an fairly typical young St Petersburg resident with an interest in Islam as well as pop music and martial arts but no obvious links to militants.
The explosion in the middle of Monday afternoon occurred when the train was in a tunnel deep underground, amplifying the force of the blast. The carriage door was blown off, and witnesses described seeing injured passengers with bloodied and blackened bodies.
State investigative authorities said fragments of the body of the suspect had been found among the dead, indicating that he was a suicide bomber.
Russia has been on alert against attacks in reprisal for its military intervention in Syria.
Russian Foreign Minister Sergei Lavrov said it was cynical to say the bombing in St Petersburg was revenge for Russia's role in Syria. He said the attack showed that Moscow needed to press on with its fight against global terrorism.
A page on social media site VKontakte, the Russian equivalent of Facebook, belonging to someone with the same name and year of birth as Jalilov, included photos of him relaxing with friends in a bar, smoking from a hookah pipe. He was dressed in jackets and a baseball cap.
President Vladimir Putin, who was visiting St Petersburg at the time of the blast, went to the site late on Monday.
The Kremlin said it was "noteworthy" that Putin had been in the city.