There was a rather scathing study on it here. [1] In short the government's own internal investigations realized the obvious answer, a mass hysteria event, was the most likely culprit. But politics got involved and so then they classified reports mentioning this, and released reports suggestive of foreign adversaries attacking people with some mystery weapon, even though their own internal reports put the chances of that at basically zero. So they then set out so study and solve the issue, unsurprisingly finding nothing. The paper's concluding paraph is brutal:
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Over the course of their 6-year investigation into “Havana Syndrome” U.S. officials expended considerable human capital and financial resources going down a rabbit hole searching for exotic explanations. Instead of finding secret weapons and foreign conspiracies—they found only rabbits. For in the end, prosaic explanations were determined to be the cause of the events in Cuba and its subsequent global spread. That is the lesson of “Havana Syndrome”—follow the science.
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Over the course of their 6-year investigation into “Havana Syndrome” U.S. officials expended considerable human capital and financial resources going down a rabbit hole searching for exotic explanations. Instead of finding secret weapons and foreign conspiracies—they found only rabbits. For in the end, prosaic explanations were determined to be the cause of the events in Cuba and its subsequent global spread. That is the lesson of “Havana Syndrome”—follow the science.
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[1] - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10913303/