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 10:15 a.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.
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.