From [1] AFP:

A blast outside a subway station in central Moscow that killed eight people was caused by a female suicide bomber, Russian news agency Interfax quoted the federal security service FSB as saying.

Earlier the interior ministry had said several theories were being considered, among them a suicide bomber but also an explosion in a car parked outside the station.

Police meanwhile had spoken of a bomb filled with bolts and other metal objects that had been placed in or under a car. But the security source quoted by Interfax gave only a female suicide bomber as an explanation for the blast.