An unidentified explosive device detonated Monday inside a passenger train in St. Petersburg, Russia, killing at least 11 people and injuring 39 others, CNN reported.
Russia's National Anti-Terrorist Committee told the BBC the blast occurred between Sennaya Ploshchad and Tekhnologichesky Institut stations.
Hours after the initial blast, the Associated Press reported that Russian officials had located and defused a second bomb in a separate St. Petersburg metro station.
During a meeting with Belarus President Alexander Lukashenko, Russian President Vladimir Putin said that investigators are already trying to determine what caused the explosion.
"I have already spoken to the head of our special services, they are working to ascertain the cause," he said, according to the BBC.
Photographs taken in the Sennaya Ploshchad metro station in the aftermath of the explosion show thick smoke choking the platform as surviving passengers hurry for the exits.
This story is breaking and will be updated.