Doctors discovered that U.S. workers who reported strange sounds and physical symptoms while working at the U.S. Embassy in Havana, Cuba, have brain abnormalities.
The finding helps to rule out suspicion that "sonic attacks" were used against the workers in early 2017. At least some of the 24 workers -- the U.S. government has not revealed whether brain abnormalities were found in all of them -- suffered damage to white matter tracts that facilitate communication between certain areas of the brain.
Doctors are still unsure how the white matter changes connect to some of the workers' reported symptoms, which include hearing, vision, balance and memory loss, The Associated Press reports.
Secretary of State Rex Tillerson announced on Dec. 6 that he's "convinced these were targeted attacks." The U.S. is still unsure who to suspect, as a few Canadian staffers also fell ill.
Most of the workers who were sick have recovered and since returned to work. About one quarter of the reported long-term symptoms are still affecting them today.
Doctors treating the workers refused to talk to AP, but reports say they are treating it like an unknown disease.
Tensions between the U.S. and Cuba have been steadily rising, which has had a negative effect on the ability of scientists from both nations to collaborate. The Cuban government has denied the workers' symptoms were from any sort of planned attack, calling the accusations "deliberate lies."
On Dec. 5, Cuban scientists declared that the workers were likely suffering from "collective psychogenic disorder," meaning there was never any real threat. Some U.S. scientists had made similar diagnoses, reports Science.
"We have devoted months to this work, but we have not found any evidence that could substantiate [the U.S.] claims," said Antonio Paz Cordovez, one of the Cuban scientists investigating the incident and the president of the Cuban Society of Otorhinolaryngology.
The scientists speculated that the then-recent election of Donald Trump contributed to the workers' anxiety over being attacked. They added that they would be willing to adjust their conclusions should new information come out.
U.S. officials have now determined the most common noises heard by the workers were a high-pitched chirping sound and, less commonly, a low-pitched hum, according to AP. The sound was loud enough to awaken some of them.
Vibrations sometimes accompanied the sounds. It reportedly felt like puffs of air felt inside of a car with its window rolled down.
The most affected victims felt symptoms of illness, such as blurred vision, within the first 24 hours. Evidence of white matter abnormality gives doctors more evidence to look at, but doesn't yet answer the question of precisely what occurred.