A new report has revealed a likely cause of disturbing medical symptoms some U.S. embassy employees in Cuba and China began experiencing in 2016. The report by the National Academies of Sciences, Engineering, and Medicine said “microwave energy” could be responsible but much remains unknown about the bizarre cases. Known as Havana syndrome, the affliction has usually started with embassy staff hearing loud noises, followed by pain in one or both ears or the head, head pressure or vibration, dizziness, tinnitus, visual problems, vertigo, and cognitive difficulties.

Although researchers said there was no medical history of incidents like these, their conclusion was supported by reports from “more than a half-century ago and over the subsequent decades by Western and Soviet sources.” The scientific, medical and government officials who worked on the report weren’t able to determine whether the “microwave energy” was deployed as an attack or who may have been responsible.