"There is no doubt that it was a terrorist attack. The explosion was caused by a device that lay on the floor of the carriage in a bag or a parcel," he told journalists.
According to Shantsev, by now 30 killed have been reported.
The train operation on the Zamoskvoretskaya line where the explosion occurred will be resumed in three or four hours, after investigators have finished their work on the scene of the tragedy, Shantsev added.
According to information RIA Novosti received from the Russian Federal Security Service, on the scene of the accident mining experts are collecting physical evidence and are looking for parts of the explosive device. They are to establish its type and weight and the way it was triggered.
However, the Service does not rule out that it was "an accidental explosion of a blasting material".