ST. PETERSBURG: A bomb blast tore through a subway train in Russia's second-largest city Monday, killing 10 people and injuring about 40 as President Vladimir Putin visited the city, authorities said. Hours later, police found an unexploded device in one of St. Petersburg's busiest subway stations, sending a wave of anguish and fear through Putin's hometown.

There was no immediate claim of responsibility for the attack Monday, but Russian trains and planes have been targeted repeatedly by Islamic militants, mostly connected to the insurgency in Chechnya and other Caucasus republics. The last confirmed attack was in October 2015 when Islamic State militants downed a Russian airliner heading from an Egyptian resort, killing all 224 people on board.

The Dec. 25 crash of a Russian plane carrying Red Army Choir members near the southern city of Sochi is widely believed to have been due to a bomb, but no official cause has been stated for the crash that killed 92 people.

The blast Monday hit the St. Petersburg train it traveled between stations about 2:20 p.m. The driver chose to continue on to the next station, Technological Institute, a decision praised by Russia's Investigative Committee as aiding evacuation efforts and reducing the danger that passengers would die by trying to walk along the subway's electrified tracks.

After a few hours of differing casualty tolls, Health Minister Veronika Skvortsova said 10 people died from the blast. City health authorities said 43 others were hospitalized.

Witnesses said the blast spread panic among passengers, who ran toward the exits of the station, which is 40 meters (130 feet) underground.

"Everything was covered in smoke, there were a lot of firefighters," Maria Smirnova, a student on a train behind the one where a bomb went off, told the Dozhd television channel. "Firefighters shouted at us to run for the exit and everyone ran. Everyone was panicking."

The entire St. Petersburg subway system, which serves some 2 million riders a day, was shut down and evacuated. Russia's National Anti-Terrorist Committee said security was immediately tightened at all of the country's key transportation sites, and Moscow officials said that included the subway in the Russian capital.

Putin, speaking Monday on television from Constantine Palace in the city, said investigators were looking into whether the explosion on the train was a terror attack or if it had some other cause. He offered his condolences to the families of those killed.

Within two hours of the blast, Russia authorities had found and deactivated another bomb at a separate busy St. Petersburg subway station, Vosstaniya Square, the anti-terror agency said. That station is a major transfer point for passengers on two lines and serves the railway station from which most trains to Moscow depart.

Russian law enforcement agencies confirmed the Vosstaniya Square device was rigged with shrapnel and the Interfax news agency said it contained up to 1 kilogram (2.2 pounds) of explosives.

Social media users posted photographs and video from the Technology Institute subway station 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 but was no immediate information on whether any foreigners were among the victims Monday.

Nataliya Maksimova, who was running late for a dentist appointment, entered the subway near the explosion site shortly after the blast.

"If I hadn't been running late, I could have been there," she told The Associated Press.

Putin was in St. Petersburg on Monday to meet with Belarusian President Alexander Lukashenko, and went ahead with the talks after appearing on Russian television to speak about the attack.

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

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

Suicide bombings in the Moscow subway on March 29, 2010, killed 40 people and wounded more than 100 people. Chechen rebel leader Doku Umarov claimed responsibility for the attack by two female suicide bombers, warning Russian leaders that "the war is coming to their cities."

A 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.

Russian airports have also been hit by attacks. On Jan. 24, 2011, a suicide bomber blew himself up at Moscow's Domodedovo Airport, killing 37 people and wounding 180. The same airport in August 2004 saw Islamic suicide bombers board two airplanes and bring them down, killing a total of 90 people.

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