Doctors treating the U.S. Embassy victims of mysterious, invisible attacks in Cuba have discovered brain abnormalities as they search for clues to explain the hearing, vision, balance and memory damage, The Associated Press has learned.

It’s the most specific finding to date about physical damage, showing that whatever it was that harmed the Americans, it led to perceptible changes in their brains. The finding is also one of several factors fueling growing skepticism that some kind of sonic weapon was involved.

Medical testing has revealed the embassy workers developed changes to the white matter tracts that let different parts of the brain communicate, several U.S. officials said, describing a growing consensus held by university and government physicians researching the attacks. White matter acts like information highways between brain cells.

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