https://en.wikipedia.org/wiki/Russian_sabotage_operations_in...
See e.g. https://www.polskieradio.pl/395/7785/artykul/2508878,russian... (2020)
> Almost 700 schools throughout Poland were in May last year targeted by hoax bomb threats during key exams, private Polish radio broadcaster RMF FM reported.
> It cited Polish investigators it did not name as saying that a detailed analysis of internet connections and a thorough examination of the content of emails with false bomb threats turned up ties to servers in the Russian city of St. Petersburg.