The brains of 40 former staffers at the US Embassy in Cuba who developed mysterious symptoms during so-called “sonic attacks” have visible differences compared to a control group, according to a new study.

The State Department has said the employees developed what became known as “Havana Syndrome” – headaches, dizziness, nausea and other symptoms that arose when they heard penetrating, high-pitched sounds.

MRI scans from the 23 men and 17 women showed changes in brain structure and functional connectivity between different parts of the organ compared with 48 other adults, according to the study by the University of Pennsylvania.

The difference in the brains between the two groups “is pretty jaw-dropping at the moment,” lead researcher Dr. Ragini Verma, a professor of radiology at Penn, told Reuters.

“Most of these patients had a particular type of symptoms and there is a clinical abnormality that is being reflected in an imaging anomaly,” she said.

However, in findings published by the Journal of the American Medical Association, Verma and her team said it was unclear if the brain patterns directly translate into significant health problems.

Initial MRI scans of 21 embassy workers in Havana had revealed no abnormalities.

The diplomats’ health problems surfaced in 2016 after the Obama administration reopened the embassy in an effort to improve relations with Cuba.

Most of the employees were removed from the Communist island nation in 2017.

President Trump has blamed Cuba for what the State Department has called “significant injuries” suffered by the workers.

Canadian embassy employees complained of similar health issues and also were removed from Cuba, whose health officials have rejected the accusation that health attacks and brain damage caused the symptoms.

Though the study did not draw any conclusions about the cause of the symptoms, the MRIs confirm that “something happened to the brains of these people,” Verma told AFP.

“It’s not imagined. All I can say is that there is a truth to be found,” she said, adding, “Whatever happened was not due to a pre-existing condition, because we test for that.”

Some of the people affected have recovered and returned to work, but others are still undergoing rehab, Verma said.

The new study was met with some skepticism.

“Finding evidence of brain change doesn’t provide evidence of brain injury or damage,” Dr. Jon Stone, a professor of neurology at the Centre for Clinical Brain Sciences at the University of Edinburgh, who was not involved in the study, told Reuters.

Dr. Sergio Della Sala, a professor of human cognitive neuroscience also at the University of Edinburgh, called the study “half baked,” noting that 12 of the affected workers who had a history of concussion prior to going to Cuba were included in the analyses.

“In comparison, none of the controls declared previous brain injury. This in itself could cause statistical group differences,” Della Sala said.

Skeptics also have challenged State Department assertions that some unknown weapon had targeted the workers.

For example, the sound that some believed may have caused the problems was later identified by insect experts as the mating call of the male Indies short-tailed cricket.