Rumors of some sort of Russian sonic weapon being behind the "Havana Syndrome" have been going around for years. https://www.youtube.com/watch?v=COWTBEl1rRc

Serbia has been Russia's closest ally for decades, with Russia having its side in the wars of the 90s. Perhaps such a weapon would have been provided to support the current pro-Moscow government.

Alternatively (and more likely) the Serbian government could've simply bought an LRAD device from the United States: https://en.wikipedia.org/wiki/Long-range_acoustic_device

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