At least 10 people have been killed following an explosion which tore through a train carriage at a St Petersburg metro station, with an image of the suspected attacker released.
It was initially feared there were two explosions, but it was later confirmed there was only one blast which occurred between the Sennaya Ploshchad and Tekhnologichesky Institut stations.
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The bomb was reportedly filled with shrapnel and several hours after the first blast, an unexploded device was found at a second metro station.
The National Anti-Terrorism Committee said in a statement carried by Russian agencies that it was "found and neutralised in a timely fashion" at the Vosstaniya Square station.
A CCTV image of a man believed responsible for the attack has been shown on Russian media but no one has claimed responsibility for the attack.
The grainy photo shows a middle-aged man with a beard and wearing a black hat.
Russian media outlet Fontanka said the man had entered Petrogradskaya station 20 minutes before the blast went off.
One witness claimed the bomb may have been placed in the carriage by a man carrying a briefcase.
Pictures captured the aftermath of the incident, and showed thick smoke filling the Sennaya Ploshchad metro station as bodies lay strewn across the platform.
It appears a door of one of the trains was blown off after the bomb exploded.
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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.
All metro stations across the city were closed immediately after the blast and security stepped up across the city and airports.
Russian President Vladimir Putin was in the city at the time of the explosion and offered his condolences to the injured.
He said the government was considering all possible causes for the blasts in St Petersburg's metro system, including terrorism.
"I have already spoken to the head of our special services, they are working to ascertain the cause" Mr Putin, at a meeting with Belarus President Alexander Lukashenko, 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.
Russia's state investigative committee has opened a criminal case over the metro blast on charges of a terrorist act.
The agency said it had sent a group of investigators to St Petersburg to look into the incident.
It said however that while it had opened a case under charges of terrorism, it would consider all other possible causes of the incident.
The blast caused scenes of confusion, with traffic blocked on Moskovsky Prospect, a busy throughfare as emergency vehicles rushed to the station.
"My mum was in the metro, I don't know what's happened to her, I can't get hold of her," one woman, Natalia, told AFP outside the station as she was trying to make a phone call on her mobile.
Pensioner Vyacheslav Veselov told AFP he had seen four bodies at the Technological Institute station.
"A station attendant in tears called on the men to help carry the bodies," he said.
Russian President Vladimir Putin was in the city at the time of the explosion and offered his condolences to the injured.
He said the government was considering all possible causes for the blasts in St Petersburg's metro system, including terrorism.
"I have already spoken to the head of our special services, they are working to ascertain the cause" Mr Putin, at a meeting with Belarus President Alexander Lukashenko, 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.
Witnesses reported people were bleeding on the train and platform, The Guardian reports.
"People were bleeding, their hair burned. We were told to move to the exit, because the movement stopped," one person said.
"People just fled. My girlfriend was in the next car that exploded. She said that she began to shake. When she came out, she saw that people were mutilated."
Russia has been on particular alert against Chechen rebels returning from Syria and wary of any attempts to resume attacks that dogged the country several years ago.
With AFP