In the most specific finding to date about physical damage, AP reports that 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.

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.

As AP details, 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.

Whatever it was that harmed the Americans, it led to perceptible changes in their brains.

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. The United States refers to “specific attacks” but says it doesn’t know who’s behind them. A few Canadian Embassy staffers also got sick.

The Cubans have urged the U.S. to release information about what it’s found. FBI investigators have spent months comparing cases to pinpoint what factors overlap.

U.S. officials told the AP that investigators have now determined:

The most frequently reported sound patients heard was a high-pitched chirp or grating metal. Fewer recalled a low-pitched noise, like a hum.

Some were asleep and awakened by the sound, even as others sleeping in the same bed or room heard nothing.

Vibrations sometimes accompanied the sound . Victims told investigators these felt similar to the rapid flutter of air when windows of a car are partially rolled down.

. Victims told investigators these felt similar to the rapid flutter of air when windows of a car are partially rolled down. Those worst off knew right away something was affecting their bodies. Some developed visual symptoms within 24 hours, including trouble focusing on a computer screen.

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.