ST. PETERSBURG, Russia (AP) — At least 10 people were killed and 50 others injured Monday when an explosion ripped through a subway train in St. Petersburg, the spokesman for the city’s governor told Russian television.

President Vladimir Putin, who was visiting the city on an unrelated trip Monday, said investigators were looking into whether the explosion was a terror attack or if there might have been some other cause. He offered his condolences to the families of those killed.

Russia’s National Anti-Terrorist Committee said an unidentified explosive device went off on a train that was traveling between two stations. Maxim Liksutov, Moscow’s deputy mayor, told Interfax that Moscow authorities were tightening security on the subway in the Russian capital.

The agency that runs the subway said several stations in the northern Russian city were closed and that an evacuation was underway.

Social media users posted photographs and video from a subway station in the city center, showing people lying on the floor outside a train with a mangled door. Frantic commuters reached into doors and windows, trying to see if anyone was there and shouting “Call an ambulance!” The explosion happened between Sennaya Ploshchad and Tekhnologichesky Institut stations.

Putin was expected to hold talks with the Belarusian president later in the day.

[Featured Image: In this grab taken from AP video, Russian police officer, left, and people walk past the damaged train at the Tekhnologichesky Institut subway station in St.Petersburg, Russia, Monday, April 3, 2017. The subway in the Russian city of St. Petersburg is reporting that several people have been injured in an explosion on a subway train. (AP video via AP)]

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