American victims of mysterious attacks in Cuba have abnormalities in their brains’ white matter, according to new medical testing reported by the Associated Press. But, so far, it’s unclear how or if the white-matter anomalies seen in the victims relate to their symptoms.

White matter is made up of dense nerve fibers that connect neurons in different areas of the brain, forming networks. It gets its name from the light-colored electrical insulation, myelin, that coats the fibers. Overall, the tissue is essential for rapidly transmitting brain signals critical for learning and cognitive function.

In August, US authorities first acknowledged that American diplomats and their spouses stationed in Havana, Cuba, had been the targets of puzzling attacks for months. The attacks were carried out by unknown agents and for unknown reasons, using a completely baffling weaponry. The attacks were sometimes marked by bizarrely targeted and piercing noises or vibrations, but other times they were completely imperceptible.

Victims complained of a range of symptoms, including dizziness, nausea, headaches, balance problems, ringing in the ears (tinnitus), nosebleeds, difficulty concentrating and recalling words, permanent hearing loss, and speech and vision problems. Doctors have also identified mild brain injuries, including swelling and concussion.

White-matter changes are often seen following brain injuries, and the extent and duration of changes can determine cognitive impairment. That said, without brain scans and tests prior to the attacks, it’s difficult to know if the abnormalities were related to the attacks or if they are from previous injuries. None of the victims reported blows to the head, however.

US officials now report that 24 Americans were injured in the attacks but wouldn’t comment on how many showed abnormalities in their white matter. The officials told the AP that most had recovered and some were even back at work. But about a quarter of the victims reported symptoms that were either persistent or took a long time to clear up. All of the victims will likely be tracked by doctors for life.

Tensions and mystery

Doctors treating the victims were tight-lipped about their treatments and findings, the AP said. And Secretary of State Rex Tillerson expressed concern for victim privacy and about releasing medical information that could reveal the effectiveness of the attacks. Nevertheless, doctors at the University of Miami and the University of Pennsylvania are working with government agencies to write up a report on the victims’ conditions and newly developed treatment protocols. Officials told the AP that the doctors would submit their report to the Journal of the American Medical Association.

Meanwhile, FBI investigators and US intelligence agencies are still struggling to understand the mysterious attacks. The attacks were initially speculated to be “sonic attacks” based on audible noises heard by some of the victims. But scientists have largely ruled out the possibility that sounds could cause traumatic brain damage.

The AP notes that shock waves from explosions in combat have been known to cause concussion and white-matter damage in soldiers. But, of course, none of the victims reported experiencing an explosion.

The agents behind the attacks and their motivation remain unknown.

At the end of September, US officials drew down the staff at the US embassy in Havana due to the safety risks. The embassy is now operating only with emergency staff. In October, the US also expelled 15 Cuban diplomats over the issue, saying the Cuban government wasn’t doing enough to protect Americans there.

In its report Wednesday, the AP quoted Secretary Tillerson as saying:

What we’ve said to the Cubans is: small island. You’ve got a sophisticated intelligence apparatus. You probably know who’s doing it. You can stop it. It’s as simple as that.

Cuba has denied any involvement with the attacks.