Ambulance workers push a stretcher carrying an injured person at the entrance to Tekhnologichesky Institut station of the St Petersburg metro in the aftermath of an explosion which occurred in a train at 14:40 Moscow time on April 3, 2017. According to Russian National Antiterrorism Committee several people were killed in the explosion. (Photo by Anton Vaganov/TASS)





Blast victims lie near a subway train hit by a explosion at the Tekhnologichesky Institut subway station in St.Petersburg, Russia, Monday, April 3, 2017. The subway in the Russian city of St. Petersburg is reporting that several people have been injured in an explosion on a subway train. (Photo by AP Photo/DTP&ChP St. Peterburg via AP Photo)





Fire fighters, emergency service vehicles and a helicopter in Moskovsky Prospekt at the entrance to Tekhnologichesky Institut station of the St Petersburg metro in the aftermath of an explosion which occurred in a train at 14:40 Moscow timeon April 3, 2017. (Photo by Peter Kovalev/TASS)





A handout photo made available by megapolisonline.ru via VKontakte (VK) shows victims shortly after an explosion in a metro of Saint Petersburg, Russia, 03 April 2017. According to reports, at least 10 people were killed and dozens injured in an explosion in the city's metro system. The cause of the blast was not immediately known. Russia's National Anti-Terrorist Committee said that the explosions hit a train between Sennaya Ploshchad and Tekhnologichesky Institut stations, media added. (Photo by EPA/Megapolisonline.ru)





The photo taken on April 3, 2017 shows the blast site at a metro station in St. Petersburg, Russia. At least 10 people were killed, 50 injured and 7 stations were shut down after blasts. (Photo by Xinhua News Agency/Rex Features/Shutterstock)





A handout photo made available by megapolisonline.ru via VKontakte (VK) shows people in front of a damaged train door shortly after an explosion in a metro of Saint Petersburg, Russia, 03 April 2017. According to reports, at least 10 people were killed and dozens injured in an explosion in the city's metro system. The cause of the blast was not immediately known. Russia's National Anti-Terrorist Committee said that the explosions hit a train between Sennaya Ploshchad and Tekhnologichesky Institut stations, media added. An anti-terror investigation is underway. (Photo by EPA/Megapolisonline.ru)





Police and emergency services personnel carry an injured person on a stretcher outside Technological Institute metro station in Saint Petersburg on April 3, 2017. Around 10 people were feared dead and dozens injured Monday after an explosion rocked the metro system in Russia's second city Saint Petersburg, according to authorities, who were not ruling out a terror attack. (Photo by Alexander Bulekov/AFP Photo)





Health team workers and firefighters are dispatched after an explosion at a subway station in St Petersburg, Russia on April 3, 2017. At least 10 people were killed on the blast. (Photo by Sergey Mihailicenko/Anadolu Agency/Getty Images)





Blast victims lie near a subway train hit by a explosion at the Tekhnologichesky Institut subway station in St.Petersburg, Russia, Monday, April 3, 2017. (Photo by AP Photo/www.vk.com/spb_today via AP Photo)





An injured man is helped by medics outside Technological Institute metro station in Saint Petersburg on April 3, 2017. (Photo by Alexander Bulekov/AFP Photo)





An injured person stands outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg, Russia April 3, 2017. (Photo by Anton Vaganov/Reuters)





General view of emergency services attending the scene outside Sennaya Ploshchad metro station in St. Petersburg, Russia April 3, 2017. (Photo by Anton Vaganov/Reuters)





An injured person is helped by emergency services outside Sennaya Ploshchad metro station in St. Petersburg, Russia April 3, 2017. (Photo by Anton Vaganov/Reuters)





General view of emergency services attending the scene outside Sennaya Ploshchad metro station in St. Petersburg, Russia April 3, 2017. (Photo by Anton Vaganov/Reuters)





An iniured person walks outside Sennaya Ploshchad metro station in St. Petersburg, Russia April 3, 2017. (Photo by Anton Vaganov/Reuters)





General view of emergency services attending the scene outside Sennaya Ploshchad metro station in St. Petersburg, Russia April 3, 2017. (Photo by Anton Vaganov/Reuters)





An injured person is helped by emergency services outside Sennaya Ploshchad metro station in St. Petersburg, Russia April 3, 2017. (Photo by Anton Vaganov/Reuters)





Russian police officers stand guard in a street after a explosion in St. Petersburg's subway, Russia, Monday, April 3, 2017. (Photo by Evgenii Kurskov/AP Photo)





A picture shows the damaged train carriage at Technological Institute metro station in Saint Petersburg on April 3, 2017. (Photo by AFP Photo/Stringer)





Paramedics carrie a subway blast victim into an ambulance after explosion at Tekhnologichesky Institut subway station in St.Petersburg, Russia, Monday, April 3, 2017. (Photo by Alexander Tarasenkov/Interpress via AP Photo)





Men carry an injured person on a stretcher outside Technological Institute metro station in Saint Petersburg on April 3, 2017. (Photo by Alexander Tarasenkov/AFP Photo)





Medics help an injured woman outside Technological Institute metro station in Saint Petersburg on April 3, 2017. (Photo by Alexander Tarasenkov/AFP Photo)





Men carry an injured person on a stretcher outside Technological Institute metro station in Saint Petersburg on April 3, 2017. Around 10 people were feared dead on Monday after an explosion rocked the metro system in Russia's second city Saint Petersburg, according to authorities. (Photo by Alexander Tarasenkov/AFP Photo/Interpress)





People react outside Technological Institute metro station in Saint Petersburg on April 3, 2017. About ten died and dozens were injured Monday after an explosion rocked the metro system in Russia's second city Saint Petersburg, according to authorities, who were not ruling out a terror attack. (Photo by Alexander Bulekov/AFP Photo)





An injured person is helped by emergency services outside Sennaya Ploshchad metro station, following explosions in two train carriages at metro stations in St. Petersburg, Russia April 3, 2017. (Photo by Anton Vaganov/Reuters)





Women grieve near the Tekhnologichesky Institut subway station in St.Petersburg, Russia, Monday, April 3, 2017. A bomb blast tore through a subway train deep under Russia's second-largest city Monday, killing several people and wounding many more in a chaotic scene that left victims sprawled on a smoky platform. (Photo by Dmitri Lovetsky/AP Photo)





Russian President Vladimir Putin, left, lays flowers at a place near the Tekhnologichesky Institut subway station in St.Petersburg, Russia, Monday, April 3, 2017. (Photo by Dmitri Lovetsky/AP Photo)





Russian president Vladimir Putin puts flowers down outside Tekhnologicheskiy Institut metro station in St. Petersburg, Russia, April 3, 2017. (Photo by Grigory Dukor/Reuters)





Russian honor guards march past the Hero City Leningrad memorial plate during memorial service for victims of blast in St. Petersburg metro, next to the Kremlin walls in Moscow, Russia April 3, 2017. (Photo by Maxim Shemetov/Reuters)





A woman lights a candle at an entrance to Sennaya subway station after an explosion in St.Petersburg subway in St.Petersburg, Russia, Monday, April 3, 2017. A bomb blast tore through a subway train in Russia's second-largest city Monday,leaving many daed and injured, as President Vladimir Putin was visiting St. Petersburg, authorities said. (Photo by Yevgeny Kurskov/AP Photo)