WASHINGTON — Doctors treating the U.S. 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 fueling growing skepticism 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 U.S. 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 byproduct of something else that caused damage, said three U.S. officials briefed on the investigation. They weren't authorized to discuss it publicly and demanded anonymity.

Related: Cubans forcefully reject blame for U.S. diplomats' mystery ailments

The Morning Rundown Get a head start on the morning's top stories. This site is protected by recaptcha

Physicians, FBI investigators and U.S. intelligence agencies have spent months trying to piece together the puzzle in Havana, where the U.S. says 24 U.S. 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 U.S. 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. U.S. 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 U.S. counter Havana's complaint that Washington hasn't presented any evidence.

Tillerson said the U.S. 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."

Related: Response to Cuba embassy attacks frustrates victim

The case has plunged the U.S. 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, U.S. 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.