Authorities in Russia have initially identified the suspect behind the St Petersburg metro blast.
They believe a young person, believed to a 23-year-old man from Central Asia and linked with radical Islamists, was a suicide bomber and carried an explosive device onto the train in a rucksack.
Police believe the same man, not a second suspect, planted a bomb at another subway station that was defused before it went off.
The seven people were from Central Asian countries and investigators say they were recruiting for IS.
The bombing of a subway train in Russia has drawn condemnation from world leaders.
A blast has ripped through an underground train in Russia's second-largest city on Monday.