Update: Russian authorities said that the bomb blast that tore through a subway train deep under St. Petersburg killed 11 people. A second explosive device was found at Vosstaniya Square, another busy station.
News reports said police were searching for two suspects, and Russian state television showed a photo of one suspect wearing what appeared to be a skullcap characteristic of Russia's Muslim regions.
The Investigative Committee, the country's top criminal investigation body, said it had begun a probe based on the assumption that the incident was an act of terrorism — but added that other possibilities were being considered.
Within two hours of the blast, authorities found and deactivated another bomb at another busy station, Vosstaniya Square, the anti-terror agency said. That station is a major transfer point for passengers on two lines, and it serves the railway station to Moscow.
Russian law enforcement agencies confirmed the device was loaded with shrapnel, and the Interfax news agency said it contained up to 2.2 pounds of explosives.
The entire St. Petersburg subway system was shut down and evacuated, but partial service resumed after about six hours.
Security was immediately tightened at all of the country's key transportation sites, Russia's National Anti-Terrorist Committee said. Moscow officials said this included the subway in the Russian capital.
The bombing drew widespread condemnation. President Donald Trump said it was "absolutely a terrible thing." White House Press Secretary Sean Spicer said the U.S. was prepared to offer assistance to Russia.
This story was originally published on April 3, 2017, at 3:15 p.m.
An explosion ripped through a subway train in St. Petersburg on Monday, the spokesman for the city's governor told Russian television. At least 10 people were killed and 50 others were injured in the incident.
President Vladimir Putin, who was visiting the city on an unrelated trip on 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.
"Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened," he said.
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 centre, 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.