Russians currently abroad are responsible for a recent wave of false bomb threats, the head of the Federal Security Service said Thursday.

Four Russian citizens are behind the threats that led to mass evacuations in dozens of cities over the past month, FSB chief Alexander Bortnikov told journalists.

The damage from the phone hoaxes, which began on Sept. 10 and spread to 50 Russian regions, is estimated at 300 million rubles ($5.2 million), Bortnikov said.