At least 11 people have been killed and dozens wounded after a bomb blast ripped through a train carriage in the metro system of Saint Petersburg, Russia's second city, according to officials.

Russian authorities opened a criminal case over Monday's explosion on charges of a "terrorist act", but said they would also consider all other possible causes. There was no immediate claim of responsibility for the bombing.

According to Russian media reports, investigators suspect that the attack was carried out by a suicide bomber who left an explosive device at a metro station, and then boarded the train with another device in a backpack.

Ambulances and fire engines descended on the central Sennaya Ploshchad metro station after the blast at around 2:40pm (11:40 GMT).

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Russian media initially reported that there were two explosions, but officials later said that there was only one blast which happened as the train was moving between the Sennaya Ploshchad and the Tekhnologichesky Institut stations.

Hours later, police found and deactivated a second explosive device at another metro station, one of the city's busiest, sending a wave of fear across St Petersburg.

After a few hours of differing casualty tolls, the National Anti-Terrorism Committee said the death toll was 11, with another 45 people being treated for wounds in hospitals.

A huge hole was blasted in the side of the carriage, with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage.

"Everything was covered in smoke, there were a lot of firefighters," Maria Smirnova, a student on a train behind the one with the bomb, told a Russian TV station. "Firefighters shouted at us to run for the exit and everyone ran. Everyone was panicking."

Social media users also posted photographs and video from the scene of the blast, showing people lying on the floor and a train with a mangled door nearby.

Russian media reports that there were 2 blasts at St.Petersburg metro https://t.co/rLHb8k6P07 pic.twitter.com/DUaVJR17Wg — Liveuamap (@Liveuamap) April 3, 2017

Russian President Vladimir Putin, who was visiting St Petersburg, his hometown, to attend an economic forum, offered his condolences to the victims' families.

"I have already spoken to the head of our special services. They are working to ascertain the cause [of the blast]," the Russian president said.

"The causes are not clear. It's too early. We will look at all possible causes, terrorism as well as common crime," he added.

He later laid flowers outside the station where the damaged train arrived after the explosion.

Authorities in St Petersburg closed all of the city's metro stations following the blast. About three million people in Russia's second-largest city use the metro daily.

"We also understand that security is being beefed up on the Moscow metro system and other transportation hubs in big Russian cities," Al Jazeera's Rory Challands, reporting from the capital, said.

"The Russians are preparing themselves for a potential series of attacks and they are doing what they can at the moment to try to prevent any more incidents like this from happening."

For years, Russian cities, including their transportation systems, had faced bomb attacks from Chechen fighters and other groups in the Caucasus, but gradually they decreased.

At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains.

A suicide raid on the capital's Domodedovo airport that was claimed by fighters from the North Caucasus killed 37 people in January 2011.

Security analysts pointed out recently that the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group had been threatening Russia, and had claimed responsibility for an attack in Chechnya that killed six Russian soldiers.

Russia is a staunch ally of Syria's President Bashar al-Assad and has been bombing rebel-held areas since September 2015.

According to monitoring groups, Russian air raids in Syria have killed more than 10,000 people, including nearly 4,000 civilians.

Simon Mabon, a lecturer in international relations at Lancaster University, told Al Jazeera that the blast in St Petersburg had the "hallmarks of a number of different groups, but the ultimate goal is to cause fear and strike at the heart of the Russian state".