Up to half a million people have been evacuated from 1,000 buildings across 80 Russian cities after a spate of bomb hoaxes that began earlier this month, a source in the security services told the state-run TASS news agency on Friday.

Previous estimates put the number of evacuations from malls, schools and transport hubs following a wave of anonymous bomb threats that started on Sept. 10 at 200,000 people.

The bomb scares culminated on Thursday with the evacuation of the Russian tech giant Yandex’s Moscow headquarters shortly after President Vladimir Putin’s visit.

The unnamed source told TASS that the bomb threats that show no signs of letting up have in total affected 80 cities, including Moscow.

“More than 1,000 sites were inspected because of false reports of the threat of an explosion, about 400,000 people were evacuated,” the source said.

An unnamed Interior Ministry source floated the theory this week that hackers based in Brussels could be responsible for the mass bomb scares.

Emergency Situations Minister Vladimir Puchkov was cited by the Interfax news agency on Wednesday as saying the threats were not drills but a “big and serious problem.”

The hoaxes were estimated to have cost the Russian economy around 300 million rubles ($5.2 million) as of the start of this week.