The mysterious symptoms afflicting American diplomats in Cuba may have spread to diplomats in China, but we still don’t know what’s making them sick. The US government says it’s investigating how the incidents are related, and it insists that some sort of attack is to blame in Cuba. Since so much of the information is kept confidential, scientists are in the dark.

Last year, news broke that people associated with the US Embassy in Cuba have been suffering from a vague collection of symptoms like headache, nausea, vertigo, and hearing loss. Possible explanations range from sonic weapons to an infection to mass hysteria to some sort of toxic exposure or even a drug. But the actual cause is still a mystery, Secretary of State Mike Pompeo said last week when he announced a task force to investigate. “The precise nature of the injuries suffered by the affected personnel, and whether a common cause exists for all cases, has not yet been established,” he said. Diplomats have been getting sick since 2016. So why don’t we know what’s going on yet?

Why don’t we know what’s going on yet?

Here’s what we do know: the diplomats in Cuba began getting sick after hearing strange noises or feeling odd vibrations. In February, a team of doctors at the University of Pennsylvania, who examined 21 of the 24 patients, said they suffered from a mysterious syndrome resembling a “concussion without concussion,” or a mild brain injury without any obvious signs of trauma, according to their paper published in the Journal of the American Medical Association (JAMA). But the researchers said in a podcast that they were pretty certain that the cause wasn’t a device using sound to cause injury (also known as a sonic weapon).

Now, these symptoms seem to be surfacing in China. In May, Pompeo and Chinese foreign minister Wang Yi discussed “a serious medical incident” affecting an American diplomat. Then, last week, The New York Times reported that the family of another employee at the American consulate in Guangzhou, China, was evacuated after he and his wife experienced similar, vague symptoms like nausea, headaches, and insomnia. (The State Department did not answer an emailed inquiry about whether US diplomats have traveled from Cuba to China.)

So, what’s going on? We don’t know, in part because these types of health investigations take time, says Sergio Della Sala, a neuropsychologist at the University of Edinburgh. “It is pretty common when something strange happens that it takes time to get to the bottom of it,” he says. But, it also takes evidence, and that’s been in short supply, Della Sala says. “All we know is what we have read in interviews, which count very little in terms of evidence, and then this paper in JAMA,” says Della Sala. “And the data in the JAMA paper are so flimsy, to verge on the absurd.”

“When something strange happens, it takes time to get to the bottom of it.”

In the JAMA study, a team of doctors put the patients associated with the US embassy in Cuba through a battery of tests. They assessed things like the patients’ thinking, mood, vision, eyesight, balance, and hearing, and they also imaged the patients’ brains. Most of the brain images were pretty typical, the study reports. But the team found that some of the patients had trouble concentrating and focusing their eyes. These symptoms could be signs of a new disorder, the team speculated, triggered by something they all were exposed to in Havana.

But much of the raw data wasn’t published along with the paper, Della Sala writes in a commentary published in the journal Cortex. That makes sense. This is confidential medical information, of government employees and their families, no less. That means that other researchers can’t evaluate the analyses or conduct their own investigations. The Verge emailed the lead and senior authors of the JAMA paper multiple times for comment; a University of Pennsylvania spokesperson told The Verge in an email that the authors were not available, adding: “We are continuing to work with the Department of State to evaluate and treat personnel who have reported audible phenomena experiences. We are not able to provide specifics about different patient groups at this time.”

Heather Nauert, a spokesperson for the State Department, said in an emailed statement to The Verge that the agency is conducting “medical screenings” of personnel in China. “US medical professionals will continue to conduct full evaluations to determine the cause of the reported symptoms and whether the findings are consistent with those noted in previously affected government personnel or possibly completely unrelated,” Nauert said. “The State Department has been and will continue to be diligent and transparent in its response to our employees’ concerns.”

“It’s been shrouded in mystery.”

The JAMA paper is one of the few pieces of scientific data that’s been shared with scientists in Cuba, the people on the ground who are best equipped to suss out what might be going on there. “We think that this could be clarified if a serious discussion is held and we exchange the information we have,” says Mitchell Joseph Valdés-Sosa, general director of the Cuban Neuroscience Center. All he and his colleagues have had to go on is a one-page medical summary with no test results or data included, he says. (The State Department did not reply to an emailed inquiry about the information shared with Cuba.)

It also doesn’t help that for months, the State Department has been characterizing these illnesses as the results of an attack, according to Valdés-Sosa. “Once you say there’s been an attack, you can’t back up,” he says. “Would they be willing to recognize that they’ve made a mistake? They’ve got themselves into a corner, and they don’t know how to pull out.”

Valdés-Sosa thinks that the best way forward is to collaborate and share data with the US, China, and Canada. (Multiple Canadian diplomats were also struck by a similar illness, according to The Guardian.) “Open scientific discussion of this issue could probably clarify it,” Valdés-Sosa says. “It’s been shrouded in mystery.”