This image provided by the American Medical Association in July 2019 shows the amount of differences between brain scans of patients, U.S. diplomats who developed concussion-like symptoms after working in Cuba, and a control group. Between late 2016 and May 2018, several U.S. and Canadian diplomats in Havana complained of health problems from an unknown cause. One U.S. government count put the number of American personnel affected at 26. (American Medical Association via AP) Associated Press

CHICAGO (AP) — New research found perplexing differences in the brains of U.S. diplomats who say they developed concussion-like symptoms after working in Cuba.

The findings published Tuesday only heighten the mystery of what may have happened to the workers.

The imaging tests showed the workers had less white matter than a comparison group of healthy people and other structural differences. But researchers say they can't explain the brain differences.

Between late 2016 and May 2018, several U.S. diplomats reported symptoms, some after hearing odd high-pitched sounds. Reported symptoms included balance problems, sleep and thinking difficulties, headaches and other complaints.

Cuba has denied any attack.

The study involved 40 people, including family members and others.