An investigation is underway in St Petersburg after an explosion on the underground metro system claimed 11 lives and injured dozens more yesterday.
Russian President Vladimir Putin, who was in the city at the time of the attack, has visited the scene and laid flowers at a makeshift shrine to the victims.
"Law enforcement agencies and intelligence services are doing their best to establish the cause and give a full picture of what happened," he said.
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There are conflicting reports as to how the attack was carried out. Law enforcement agencies initially said they were searching for two suspects.
The BBC reported that the Russian security service had named the bomber as Kyrgyzstan national Akbarzhon Jalilov, who was born in 1995. However, another news agency, Tass, said a woman may have also been involved.
So far, there have been no claims of responsibility and President Putin has said investigators are exploring a number of possibilities.
World leaders have condemned the attack. German Chancellor Angela Merkel described it as a "barbaric act" and US President Donald Trump said it was "a terrible thing".
Initial reports suggested there had been two explosions at Sennaya Ploshchad and Tekhnologichesky Institut stations, but it was later confirmed there was only one explosion as the train travelled between the stations.
Authorities in St Petersburg have declared three days of mourning.
At least ten killed in Russia metro 'terror' blast
4 April
At least ten people have been killed and more than 50 injured in an explosion on the St Petersburg metro this afternoon.
Early reports indicate there were two blasts inside a train travelling between the Sennaya Ploschad and Tekhnologichesky Institut stations. It is thought they were caused by a nail bomb.
Russia's prosecutor general described it as a terrorist attack.
Images posted on social media showed injured people lying close to a mangled carriage in Sennaya Ploschad station, a busy intersection for five metro lines.
Other photos showed the hole where a carriage door had been blown off.
Interfax news agency said the explosions involved a device filled with shrapnel, reports the BBC.
A witness told Gazeta.ru: "There was a thundering clap followed by a strong smell of smoke. We all moved to the opposite end of the carriage. People were jammed together and two women passed out. The train was still moving, it didn't stop."
The entire metro system was shut down as emergency services attended to the injured.
Russian President Vladimir Putin, who is in his hometown of St Petersburg for talks with Belarussian President Alexander Lukashenko, said investigators were looking into possible terror links.
"The reasons for the explosion are unknown so it's too early to talk about it. The investigation will show what happened," he said.
"Naturally, we always consider all options, both domestic and criminal, primarily incidents of a terrorist nature."
Russia has been the target of attacks by Chechen militants in the past, reports The Independent. In 2010, two female suicide bombers killed at least 38 people in rush hour attacks on Moscow's metro system.
The Guardian adds that since Russia entered Syria, "a number of Islamic State propaganda outlets have said the country would be a target".
