More than 20,000 Muscovites were evacuated Friday from some 100 buildings in the capital, including schools, government buildings, and train stations, the RBC news outlet reported Monday.

A wave of bomb hoaxes since Sept. 10 across 50 regions has forced hundreds of thousands to flee public buildings in major towns and cities throughout Russia.

Alexander Bortnikov, head of the Federal Security Service (FSB) Russia’s domestic intelligence agency said Friday that four Russian citizens abroad are using online calling services to make the anonymous threats.

Earlier Friday, 1,500 people were forced from five schools in Moscow after anonymous phone-calls claiming explosives had been planted. Some 6,000 people evacuated 41 schools in the Moscow suburbs.

The Moscow regional Duma as well as a shopping mall and the Marriott Grand Hotel were also cleared.