ST. PETERSBURG, Russia (AP) — A bomb blast tore through a subway train in Russia's second-largest city Monday, killing 11 people and injuring about 40 as President Vladimir Putin was visiting St. Petersburg, authorities said. Hours later, police found an unexploded device in one of the city'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. The Interfax news agency said law enforcement agencies were searching for two suspects, and state TV showed a photo of one possible suspect wearing what appeared to be a skullcap characteristic of Muslim regions in the former Soviet Union.
Russia's top criminal investigative body, the Investigative Committee, said it was investigating the blast as a terrorist attack, but said it was also considering other motives.
The explosion rocked the train as 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 the Investigative Committee as aiding evacuation efforts and reducing the danger of more deaths from passengers trying to walk along the subway's electrified tracks.
The National Anti-Terrorism Committee said the death toll was 11, with another 45 people being treated for wounds in hospitals.
Witnesses said the explosion 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 the 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; Moscow officials said that included the subway in the Russian capital.
Putin, speaking Monday on television from Constantine Palace in St. Petersburg, said investigators were looking into whether the explosion 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 train 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 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.
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.
Vasilyeva reported from Moscow. Vladimir Isachenkov and Jim Heintz in Moscow contributed to this report.