The nature of the damage in the railcar in the St. Petersburg metro, where an explosion occurred on Monday, indicates that the explosive device had been left on the train before it was departing the Sennaya Ploshchad station towards the Tekhnologichesky Institut station, a source familiar with the situation told Interfax.

"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.

Investigators are currently giving priority to the extremist or nationalistic trace theory behind the blast, he said.