Eleven people were killed in a blast on the St. Petersburg metro yesterday, the Russian health ministry said, in what authorities described as a terrorist attack.

President Vladimir Putin, who had been in St Petersburg earlier in the day, said all causes were being investigated, including terrorism. Russian Prime Minister Dmitry Medvedev described the explosion as a “terrorist act.”

An explosion tore through a train as it was traveling between two stations in Russia’s second-biggest city, injuring dozens more. A second device was found and defused at another station, Russia’s Anti-terrorist Committee said. At press time, no group had claimed responsibility for the attack, which led to the shutdown of the entire metro system in St. Petersburg.

The blast happened as the train was traveling in a tunnel from Sennaya Ploshchad to Tekhnologichesky Institute stations in the center of the city. In the confusion, first reports suggested there were two blasts.

Photographs show the facade of one of the cars ripped off, while others showed passengers running from the Tekhnologichesky Institute station as it filled with smoke. Victims said they helped each other escape the train.

Bodies were seen strewn across a station platform outside the train. Bandaged and bloodied victims were carried out of the station by rescuers. Putin expressed his condolences to the victims and was talking to the FSB security services about the investigation, according to state media. He said he was not ruling out any causes.

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“The reasons for the explosion are unknown, so it’s too early to talk about it. The investigation will show what happened,” Putin said, in a meeting with Belarussian President Alexander Lukashenko.

“Naturally, we always consider all options, both domestic and criminal, primarily incidents of a terrorist nature.” Kremlin spokesman Dmitry Peskov said Putin had been in St. Petersburg speaking at a media event earlier yesterday. Viktor Ozerov, Chairman of the defense committee in the Federation Council, said that the choice of St. Petersburg as a target may have been tied to Putin’s visit there.

“The choice of the place and the timing of these blasts is not accidental. The president of Russia is in (St. Petersburg), the media forum is taking place there, there are many journalists,” Ozerov said.

Medvedev said vicitms would be provided with “all necessary assistance.” He said in a Facebook message: “My most sincere condolences to the families and friends of the victims of the explosion.”

Russia was once a hotspot for terror attacks but the country has experienced relatively few in recent years.  In December 2013, a suicide bombing at a train station in Volgograd killed at least 16 people. The following day, in the same city, a suicide bombing on a trolley bus killed 14 people.

In 2010, two female suicide bombers blew themselves up at two Moscow metro stations, killing 40. They were linked to the Chechen insurgency. And in 2002, Chechen rebels killed 170 people in a theater hostage situation in the capital, Moscow.