Bortnikov elaborated that six of the arrestees were detained in Moscow and two in St. Peterburg. The terrorists had a large amount of weapons and munitions with them, all of which were seized by security officers.
"A search operation for the organizers and possible accomplices is underway," Bortnikov said.
In 2016, Russian security officers prevented a total of 16 terrorist acts in nine Russian cities, eliminating 46 international terrorist cells, Bortnikov said.
The FSB chief called for additional border measures to prevent terrorist infiltration, adding that the St. Petersburg attack demonstrated how the current measures had failed to fully meet possible threats.
"Most members of terrorist cells are CIS nationals who come to Russia as part of the immigrant workforce," Bortnikov said.
On April 3, an explosive device blasted in St. Petersburg in a subway train. Another blast that was supposed to take place on the Ploshchad Vosstaniya station was prevented by bomb technicians. The explosion resulted in at least 14 deaths. Following the incident, Russia's Investigative Committee has launched a criminal case over a terrorist act but the investigators are also looking into other versions.