A group of American diplomats stationed in Havana appear to have symptoms of concussion without ever having received blows to their heads, medical experts have found. The diplomats originally were said to have been victims of a "sonic attack," a possibility that the Federal Bureau of Investigation reportedly ruled out. The experts' report, published late Wednesday in the journal JAMA, does not solve the mystery, instead raising even more questions about what could have caused the brain injuries. The incidents occurred in 2016, when 18 of the 21 affected diplomats reported they heard strange sounds in their homes or hotel rooms. All but one reported immediate symptoms: headache, pain in one ear, loss of hearing. Days or weeks later, other symptoms emerged. The State Department asked researchers at the University of Pennsylvania to investigate. Their report confirmed neurological problems in the diplomats, including signs of what appear to be concussions. The study's lead author, Dr. Douglas H. Smith [said], "This is ... concussion without blunt head trauma." Like concussion patients, some of the diplomats improved on their own, while others recovered only after therapy. Dr. Smith and his colleagues do not think audible sound caused the injuries. Perhaps, they speculated, a device that produced another sort of harmful energy also produced an audible sound. Low-frequency infrasound, high-frequency ultrasound and microwaves have all been shown to damage the brain, the researchers noted.