What do you think is the cause of "Havana syndrome"?

“Havana syndrome” refers to a series of debilitating symptoms experienced by U.S. officials in Havana, Cuba (but similar incidents have reported in other places including China and even the Washington D.C. area) that have defied explanation. What it has in common is it always seems to be government employees and the attacks seem to target the brain.

What do you think is going on? Is it an actual radioactive or sonic weapon? Is it psychosomatic or mass hysteria? What are your theories?