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.

Loud, mysterious sounds followed by hearing loss and ear-ringing had led investigators to suspect “sonic attacks.” But officials are now carefully avoiding that term. The sounds may have been the byproduct of something else that caused damage, said three U.S. officials briefed on the investigation. They weren’t authorized to discuss it publicly and demanded anonymity.