FILE PHOTO - Director of Russian Federal Security Service (FSB) Alexander Bortnikov attends the annual Moscow Conference on International Security (MCIS) in Moscow, Russia, April 26, 2017. REUTERS/Maxim Shemetov

MOSCOW (Reuters) - The head of Russia’s FSB security service said on Thursday Moscow knew the identities of four Russian citizens responsible for a wave of bomb hoaxes.

The anonymous phone calls have caused huge disruption at shopping centers, train stations and public buildings across Russia since last month.

Alexander Bortnikov, the head of the FSB, told reporters on Thursday that authorities knew the identities of those responsible, Russian news agencies reported. He said they were four Russian citizens based abroad with “accomplices” inside Russia.

Bortnikov was also quoted as saying that Moscow was working with foreign authorities to try to locate and extradite the hoaxers, who he said had used IP phones to call in their threats, making it hard to trace them.