Dozens of buildings including schools and malls in more than 12 Russian cities have been evacuated since Sunday after authorities received a series of anonymous bomb threats, the Interfax news agency reports.

Interfax cited a law enforcement official in Chelyabinsk as saying that the evacuations were prompted by bomb scares across cities in Russia. “There’s reason to assume this was all organized abroad,” the official was cited as saying.

The wave of evacuations began in Omsk early on Sunday, when law enforcement cleared several cinemas, schools, malls and City Hall, Interfax reports.

That evening, eleven buildings were also cleared in Ryazan after authorities received phone calls warning a bomb had been planted in several malls. On Monday, similar actions were taken in Chelyabinsk and Kopeisk, Ufa and Stavropol, which received as many as 42 bomb scares, Interfax reports.

The bomb scares continued on Tuesday when all schools in Perm were evacuated, alongside several malls and a bus and train station, Interfax reports, citing local journalists.

Government buildings were also ordered to be emptied in Yuzhno-Sakhalinsk, and bus stations were evacuated in Novosibirsk and Yekaterinburg, according to Meduza.

Meduza cited law enforcement in some regions as saying the evacuations were the result of drills.

Russia was rocked by a series of small-scale attacks and arrests this year after a bomber detonated a homemade device on the St. Petersburg metro in April, killing 16 people and wounding dozens.