WASHINGTON, United States (AP) — Doctors treating the US Embassy victims of mysterious, invisible attacks in Cuba have discovered brain abnormalities as they search for clues to explain the hearing, vision, balance and memory damage, The Associated Press has learned.

It’s the most specific finding to date about physical damage, showing that whatever it was that harmed the Americans, it led to perceptible changes in their brains. The finding is also one of several factors fuelling growing scepticism that some kind of sonic weapon was involved.

Medical testing has revealed the embassy workers developed changes to the white matter tracts that let different parts of the brain communicate, several US officials said, describing a growing consensus held by university and government physicians researching the attacks. White matter acts like information highways between brain cells.

Loud, mysterious sounds followed by hearing loss and ear-ringing had led investigators to suspect “sonic attacks.” But officials are now carefully avoiding that term. The sounds may have been the by-product of something else that caused damage, said three US officials briefed on the investigation. They weren’t authorized to discuss it publicly and demanded anonymity.

Physicians, FBI investigators and US intelligence agencies have spent months trying to piece together the puzzle in Havana, where the US says 24 US government officials and spouses fell ill starting last year in homes and later in some hotels. Secretary of State Rex Tillerson said Wednesday he’s “convinced these were targeted attacks,” but the US doesn’t know who’s behind them. A few Canadian Embassy staffers also got sick.

Doctors still don’t know how victims ended up with the white matter changes, nor how exactly those changes might relate to their symptoms. US officials wouldn’t say whether the changes were found in all 24 patients.

But acoustic waves have never been shown to alter the brain’s white matter tracts, said Elisa Konofagou, a biomedical engineering professor at Columbia University who is not involved in the government’s investigation.

“I would be very surprised,” Konofagou said, adding that ultrasound in the brain is used frequently in modern medicine. “We never see white matter tract problems.”

Cuba has adamantly denied involvement, and calls the Trump administration’s claims that U.S. workers were attacked “deliberate lies.” The new medical details may help the US counter Havana’s complaint that Washington hasn’t presented any evidence.

Tillerson said the US had shared some information with Havana, but wouldn’t disclose details that would violate privacy or help a perpetrator learn how effective the attacks were.

“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,” Tillerson said. “It’s as simple as that.”

The case has plunged the US medical community into uncharted territory. Physicians are treating the symptoms like a new, never-seen-before illness. After extensive testing and trial therapies, they’re developing the first protocols to screen cases and identify the best treatments — even as the FBI investigation struggles to identify a culprit, method and motive.

Doctors treating the victims wouldn’t speak to the AP, yet their findings are expected to be discussed in an article being submitted to the Journal of the American Medical Association, US officials said. Physicians at the University of Miami and the University of Pennsylvania who have treated the Cuba victims are writing it, with input from the State Department’s medical unit and other government doctors.

But the article won’t speculate about what technology might have harmed the workers or who would have wanted to target Americans in Cuba. If investigators are any closer to solving those questions, their findings won’t be made public.

The AP first reported in August that US workers reported sounds audible in parts of rooms but inaudible just a few feet away — unlike normal sound, which disperses in all directions. Doctors have now come up with a term for such incidents: “directional acoustic phenomena.”

Most patients have fully recovered, some after rehabilitation and other treatment, officials said. Many are back at work. About one-quarter had symptoms that persisted for long periods or remain to this day.

Earlier this year, the US said doctors found patients had suffered concussions, known as mild traumatic brain injury, but were uncertain beyond that what had happened in their brains. Concussions are often diagnosed based solely on symptoms.

Studies have found both concussions and white matter damage in Iraq and Afghanistan war veterans who survived explosions yet had no other physical damage. But those injuries were attributed mostly to shock waves from explosions. No Havana patients reported explosions or blows to the head.