Twelve people, including a female suicide bomber, were killed and 39 wounded in Thursday's blast at a bus stop in Vladikavkaz, the capital of Russia's North Caucasus republic of North Ossetia.
"There are two main versions: destabilization of the general situation in the Caucasus and return to the Ossetian-Ingush conflict," investigation chief Alexander Bastrykin told journalists.
Opening a session on law enforcement issues in Russia on Friday, President Dmitry Medvedev said that a terrorist threat still exists in the country.
"This event has shown that a terrorist threat remains," Medvedev said. "We should investigate what has happened as quickly as possible."
North Ossetia borders the disputed Georgian republic of South Ossetia, which Russia recognized as independent in August after a brief conflict with Georgia, as well as the restive Russian regions of Ingushetia and Chechnya.
