Sixty-six people were rushed to hospital on Monday after bombs containing capsules of an unidentified gas exploded in three trade centres of Russia's second largest city St Petersburg.
According to a RIA Novosti news agency report quoting local department of the Russia's Federal Security Service (formerly KGB), 12 of those hospitalised were in serious condition.
The victims of the attack were suffering from coughing, suffocation and rapid heartbeats. Several of them are being treated at the scene of the attack, the report addded.
The Russian Emergency Situations Ministry said the attacks occurred in an outlet of the Maksidom chain. Suspicious containers with pipes were found in two other shops, which were being defused, it said.
A source in the St Petersburg Prosecutor's Office told Interfax news agency that preliminary investigations have revealed that a substance called Merkantan was used in the attacks.