The bizarre case of the invisible attacks that took place at the US embassy in Cuba earlier this year has become even stranger. Doctors treating the victims have discovered abnormalities in their brains’ white matter as they search for an explanation behind their hearing, vision, balance, and memory damage.

The findings cast more doubt over original claims that the 24 government officials and spouses were attacked by a “sonic weapon.”

The attacks, which had been ongoing for months and took place in homes and hotels, often consisted of “a deafeningly loud sound similar to the buzzing created by insects or metal scraping.” Sometimes, the sounds were audible in certain parts of rooms but inaudible just a few feet away. On other occasions, the attacks were complete imperceptible. You can listen to an alleged recording of the noise here.

In addition to problems such as dizziness, nausea, headaches, tinnitus, permanent hearing loss, and speech and vision problems, the Associated Press reports that changes to the white matter tracts, which allow different parts of the brain to communicate, are another symptom of the attacks. Officials wouldn’t say how many of the 24 victims displayed the abnormalities.

Exactly how these attacks were carried out, and who was behind them and why, is still a mystery, though Secretary of State Rex Tillerson said Wednesday he's "convinced these were targeted attacks.”

The new information means the chances a sonic weapon was used are even lower, as they have never been shown to alter the brain’s white matter tracts.

While similar white matter damage has been found in the brains of Iraq and Vietnam war veterans who survived explosions and other damage, none of the Cuba victims reported any explosions.

Cuba has denied any knowledge of the attacks, but it seems that Tillerson is skeptical of its claims. “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,” he said.