On Monday, a deadly blast in St. Petersburg occurred on the city's subway system, between the Sennaya Square and Teknologichesky Insititue stations.
An investigation was launched under Article 205 of the Russian Criminal Code, which is specifically for investigations into terrorist attacks.
"The investigators will probe other potential causes of the attack," Russian Investigative Committee spokeswoman Sveltana Petrenko said in comments after the scene had been secured.
The city of St. Petersburg immediately declared a three day period of mourning in wake of the attack. The perpetrators have not been identified and their intentions are not known.
"The authorities of the city, and the federal ones if needed, will take all necessary steps to provide help to the relatives of the killed" President Vladimir Putin said, according to the Kremlin's information service.
German Chancellor Angela Merkel offered condolences to the Russian people following the "babaric act."