An explosion Monday afternoon ripped through a subway train in the Russian city of St. Petersburg, killing at least 10 people and injuring 50 others, the spokesman for the city's governor told Russian television. The blast came as Russian President Vladimir Putin was visiting the city, his hometown.

Putin, speaking from Constantine Palace in St. Petersburg, 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.

There was no immediate claim of responsibility for the blast.

The unidentified explosive device went off at 2:20 p.m. on a train that was leaving the Technology Institute station and heading to the Sennaya Square station, Russia's National Anti-Terrorist Committee said.

The St. Petersburg subway immediately shut down all of its stations and the national anti-terrorism body said security measures would be tightened all key transport facilities across Russia. Maxim Liksutov, Moscow's deputy mayor, told Interfax that Moscow authorities were tightening security on the subway in the Russian capital.

Social media users posted photographs and video from the Technology Institute subway station in the city center, showing injured people lying on the floor outside a train with a mangled door. Frantic commuters were reaching into doors and windows, trying to see if anyone was there, and shouting "Call an ambulance!"

St. Petersburg, Russia's second-largest city with over 5 million residents, is the country's most popular tourist destination. The two stations that were the site of the blast are some of the subway's busiest.

Putin was in St. Petersburg on Monday for talks with Belarusian President Alexander Lukashenko.

"Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened," Putin said at the start of his talks with Lukashenko.

St. Petersburg governor Georgy Poltavchenko was overseeing the rescue effort.

Russian transport facilities have been the target of previous terror attacks.

Double suicide bombings in the Moscow subway in March 2010 killed 40 people and wounded more than 100 people. Chechen rebel leader Doku Umarov claimed responsibility for that attack by two female suicide bombers, warning Russian leaders that "the war is coming to their cities."

The high-speed Moscow-to-St.Petersburg train was also bombed on Nov. 27, 2009 in an attack that left 26 dead and some 100 injured. Umarov's group also said he ordered this attack.
 

AP