The police also suspect that the explosion was caused by an Improvised Explosive Device or IED.
Surveillance cameras in St Petersburg's metro system may have captured images of the person suspected of organising Monday's deadly train blast, Russian news agency Interfax quoted a source as saying.
"Images of the suspected organiser of the metro blast were captured on metro station cameras," the source said.
The explosive device may have been left in a briefcase in a metro train carriage, the source added.
At least nine people were killed and 20 were injured when an explosion tore through a train carriage in the St. Petersburg metro system on Monday.
(Reporting by Alexander Winning, Writing by Sujata Rao)
© Thomson Reuters 2017
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