Media reports said two stations still remain closed, namely: Sennaya Ploschad and Technologichesky Institute stations.
Russias Investigative Committee (IC) opened a criminal case on the explosion in the metro of St. Petersburg on Monday, an official statement released by the IC said.
An explosion of an unknown explosive device took place around 14:40 local time in a train carriage on the runway between the metro stations Technological Institute and Sennaya Ploshchad (Square) in St.Petersburg.
"Despite the fact that the criminal case has been launched under Article 205 of the Russian Criminal Code (terrorist act), the investigators will probe other potential causes of the incident," ICs spokeswoman Svetlana Petrenko said.
According to the statement, a group of experienced investigators and criminologists from the central office of the Investigative Committee was sent to St. Petersburg. Initial investigative actions are being carried out.
The prosecutors office in St. Petersburg has also begun an investigation in connection with the explosion. The prosecutor of the city Sergei Litvinenko has already arrived at the scene to coordinate the actions of law enforcement agencies, according to a statement from the Prosecutor Generals of Russia Office.
"We will do everything we can to clarify all the points that contributed to the commission of the terrorist act, so that this does not happen again in the future," TASS quoted Representative of the Prosecutor Generals office Alexander Kurennoy as saying.
Earlier in the day, Russian President Vladimir Putin said that Russian law enforcement agencies are working to determine circumstances of the explosions, and all possible causes are being considered, including terrorism.