According to latest update from Russian security forces, ten people (up to 14 according to unconfirmed reports) have been killed and at least 37 were injured when an explosion tore through a train carriage in a St. Petersburg metro tunnel on Monday. According to an update from Interfax news agency, the blast may have been caused by an explosive device hidden in a briefcase which had been left on the train before it was departing the Sennaya Ploshchad station towards the Tekhnologichesky Institut station.

"According to preliminary data, the explosion in the St. Petersburg metro system was committed not by a suicide bomber; the explosive device had most likely been left behind in the railcar before the train's departure," the source said.

Alleged photos of the IED surfaced on social media. They show a medium-sized leather bag, which apparently held a container filled with explosives, with ball bearings wrapped around it.

Interfax also said that surveillance cameras had captured images of what it called the organizers of the explosion, which hit St Petersburg as President Vladimir Putin was visiting the city. As a result, Russian security forces have issued a search warrant for two people suspected to be behind the blast.

Moments ago, Russia's Rossiya-24 television aired a picture of St. Petersburg metro terror suspect.

Russia's National Anti-Terrorist Committee also said an undetonated improvised explosive device (IED) was discovered and defused by investigators at the Ploshchad Vosstaniya Station and defused by specialists, the NAC reported. It rejected media reports which claimed that more than one explosion occurred, saying no evidence supporting this was found.

Putin, in another part of the city for a meeting with Belarus's leader, was initially cautious. He said he was considering all possible causes, including terrorism.

Investigators are currently giving priority to the extremist or nationalistic trace theory behind the blast. A terrorism probe has been launched over the blast by Russia’s Investigative Committee. The committee also praised the driver of the train for not trying to stop in the tunnel after the explosion.

“He acted right in the circumstances. The explosion happened in the tunnel between stations, but the driver took the right decision and brought it to the next station, which allowed evacuation and help to the injured to start at once. This may have prevented casualties,” the committee said.

Russian President Vladimir Putin commented that “The causes of this event have not been determined yet, so it’s too early to talk about [possible causes]. The investigation will show. Certainly, we will consider all possibilities: common, criminal, but first of all of a terrorist nature.”

All Metro stations are closed to passengers. Commuters were evacuated from the subway within about an hour of the first reports of the blast coming in. Ambulances and fire engines descended on the concrete-and-glass Sennaya Ploshchad metro station. A helicopter hovered overhead as crowds gathered to observe rescue operations.

"I appeal to you citizens of St. Petersburg and guests of our city to be alert, attentive and cautious and to behave in a responsible matter in light of events," St Petersburg Governor Georgy Poltavchenko said in an address.

As Reuters adds, an attack on Russia's old imperial capital would have symbolic force for any militant group, notably Chechen secessionists and Islamic State, which is now fighting Russian forces in Syria. Chechen militant attacks in the past have largely focused on Moscow, including an attack on an airport, a theater and in 2010 a metro train

Video from the scene of Monday's blast showed injured people lying bleeding on a platform, some being treated by emergency services and fellow passengers. Others ran away from the platform amid clouds of smoke, some screaming or holding their hands to their faces.

“I saw a lot of smoke, a crowd making its way to the escalators, people with blood and other people's insides on their clothes, bloody faces,,” St Petersburg resident Leonid Chaika, who said he was at the station where the blast happened, told Reuters by phone. "Many were crying."

A huge hole was blown open in the side of a carriage with metal wreckage strewn across the platform. Passengers were seen hammering at the windows of one closed carriage. Russian TV said many had suffered lacerations from glass shards and metal, the force of the explosion maximized by the confines of the carriage and the tunnel.

Authorities closed all St. Petersburg metro stations. The Moscow metro said it was taking unspecified additional security measures in case of an attack there. Russia has been on particular alert against Chechen rebels returning from Syria, where they have fought alongside Islamic State, and wary of any attempts to resume attacks that dogged the country several years ago.

At least 38 people were killed in 2010 when two female suicide bombers detonated bombs on packed Moscow metro trains. Over 330 people, half of them children, were killed in 2004 when police stormed a school in southern Russia after a hostage taking by Islamist militants. In 2002, 120 hostages were killed when police stormed a Moscow theater to end another hostage-taking.