Some 10 people were killed Monday in an explosion on the subway in Saint Petersburg, Russian authorities said.
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, which said “several” people were killed and injured, said an unidentified explosive device went off on a train that was traveling between two stations.
Andrei Kibitov, spokesman for the St. Petersburg governor, told Russian television 10 people were killed and 50 injured in the subway explosion.
The blast occurred at the Technological Institute metro station’s platform, a busy hub of the underground network in the center of Russia’s second-largest city.
Russian news reports said that a security camera caught a person who could be responsible for the blast.
The Interfax news agency cited an unidentified source who said the suspect in Monday’s blast might have left the explosive device in a bag. It didn’t explain why the man was believed to be the culprit.
The subway’s administration said several stations in the city were closed and that an evacuation was underway. Reports indicated that all stations in the city had been shut.
The Russian anti-terrorism committee said it had found and deactivated a bomb at another Saint Petersburg subway station.
Social media users posted photographs from the scene of the blast, showing people lying on the floor and a train with a mangled door nearby.
Russian Senator Viktor Ozerov told the Interfax agency that the explosion looked like a terrorist attack.
Following the reports, the Moscow metro also announced that it was “taking additional security measures” as required by law in such situations, according to the network’s official Twitter account.
While there was no immediate indication as to what caused the blast, Russia’s security services have previously said they had foiled “terrorist attacks” on Moscow’s public transport system by militants, some of whom were trained by Islamic State jihadists in Syria.
And Russia’s public transportation systems have been targeted by attacks in the past.
In 2013, Russia was hit by twin suicide strikes that claimed 34 lives and raised alarm over security at the Sochi Winter Olympic Games.
A bombing at the main railway station of the southern city of Volgograd killed 18 people on while a second strike hit a trolleybus and claimed 16 lives.
A suicide raid on Moscow’s Domodedovo airport that was claimed by Islamic insurgents from the North Caucasus killed 37 people in January 2011. That strike was claimed by the Caucasus Emirate movement of Islamist warlord Doku Umarov.
Russia beefed up its security over the holiday period in the wake of the attack on the Berlin Christmas market that killed 12. Authorities placed heavy trucks at road intersections to block off areas where public festivities were taking place after the attack in the German capital that was claimed by the Islamic State group.
Russia has intervened militarily to bolster Syrian President Bashar al-Assad’s forces in September 2015, turning the tables on the battlefield just as rebel forces were strengthening their hold on key areas.
Russian bombardments helped the regime retake rebel areas in the east of the northern city of Aleppo after four years of fighting.
More than 310,000 people have been killed in Syria since the conflict broke out in March 2011 with protests against Assad’s rule.