The sounds were usually high-pitched, like a buzzing of insects, the grinding of metal, or a piercing squeal of something unknown. Other times, it was a low humming. Most often it brought pressure or vibrations into the heads of the American diplomats, some of whom covered their ears out of pain. For months afterward, most reported memory problems, impaired concentration, sleep problems, and irritability.

The mysterious illness that hit the U.S. representatives in Havana, Cuba in late 2016 remains officially unexplained. But a team of specialists from the University of Pennsylvania have published their findings from a battery of tests on the 21 patients who were exposed to the phenomena in the Journal of the American Medical Association.

“These individuals appeared to have sustained injury to widespread brain networks with an associated history of head trauma,” write the doctors.

The 21 U.S. government personnel were exposed to the sounds – and strange sensations – in their homes and hotel rooms.

The initial evaluations were performed at the University of Miami. But the Penn battery of tests included a series of nearly two dozen assessments. They ranged from cognitive evaluations to mood tests, balance and vestibular exams, to vision and oculomotor trials.

The majority reported a combination of symptoms, including cognitive and memory problems, visual problems, balance difficulties, and sleep disturbances.

The assessments determined all but one had some kind of brain injury similar to a concussion. The 21 people who experienced the symptoms over a number of months have mostly improved due to rehabilitation including cognitive and physical exercises, and a gradual return to work.

But they still don’t officially know what it is.

“It appears we have identified a new syndrome that may have important public health implications,” said Douglas H. Smith, the senior author, director of Penn’s Center for Brain Injury and Repair. “None of these patients have suffered any type of blunt head trauma, yet the symptoms they describe and evaluations demonstrate are remarkably similar to those found in persistent concussion syndrome.”

However, one of the limitations mentioned in the JAMA paper indicates there may be more to the story, based on the circumstances of the diplomats falling ill, they write.

“Due to the sensitive nature of this publication, certain details typically reported in a case series of exposure were omitted, including specifics about geography, relationships between individuals, and individual demographics,” they write. “There may be additional individuals exposed while in Havana, Cuba, who have not been identified due to subtler manifestations that either resolved spontaneously or did not prompt presentation for medical treatment. Therefore, the actual number of individual exposed is known, and the relative ‘dose’ of exposure that causes acute and chronic symptoms remains unclear.”