SAINT-PETERSBURG, RUSSIA - APRIL 3: Fire fighters 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)
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SAINT-PETERSBURG, RUSSIA - APRIL 3: 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)
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SAINT-PETERSBURG, RUSSIA - APRIL 3: Health team workers 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)
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SAINT-PETERSBURG, RUSSIA - APRIL 3: People wait near the area 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)
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Emergency vehicles and a helicopter are seen at the entrance to Technological Institute metro station in Saint Petersburg on April 3, 2017. A blast hit the metro system of Russia's second city Saint Petersburg Monday, authorities said, with news agencies quoting security sources as saying that about ten people have been killed. (Photo by Valya Egorshin/NurPhoto via Getty Images)
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