On Sept. 29, the State Department recalled 21 employees of the American Embassy in Havana, Cuba. The group (along with three Canadians) were all suffering from an odd combination of symptoms: “hearing loss, dizziness, headache, fatigue, cognitive issues, and difficulty sleeping,” Secretary of State Rex Tillerson ticked off in an official statement.

On Wednesday, an upcoming report to be published in the Journal of the American Medical Association by doctors at the University of Miami, the University of Pennsylvania, and government medical experts was reported by the AP to include evidence that the 24 victims showed clear patterns of brain abnormalities.

(The Daily Beast reached out to doctors at the University of Miami’s otolaryngology department and University of Pennsylvania’s otorhinolaryngology department—the medical divisions most likely to be considering the effects of ear and brain abnormalities—for comment. The University of Miami did not return calls; a doctor at the University of Pennsylvania refused to comment.)

The symptoms were clear. The reason, though? Not so much.

That doesn’t mean people haven’t tried. Secretary of State Rex Tillerson said he’s “convinced these were targeted attacks,” but Cuba has contradicted any reports of its involvement. An alleged sonic attack on the American Embassy in Uzbekistan also raised suspicion that Russia has some involvement in the Cuban mystery. The phrase “sonic attack” caught on quickly after employees reported having heard a constant high-pitched, tinny hum emanating from certain corners of the embassy that would become inaudible if a person moved just a few feet away.

Experts remain stumped, particularly because hearing damage such as what the victims are reporting requires a loud bang. Russell Gore, M.D., director of the complex concussion program at Shepherd Center for spinal cord and brain injury rehabilitation in Atlanta, said that what the AP reports the victims heard—a high-pitched chirp or grating metal or a low-pitched noise, like a hum—aligns somewhat with what experts know about the sounds from explosions that can lead to white matter injuries.

“What’s so interesting is that we know, mostly from the military community, that brain injuries can be caused by blasts,” Gore said. Victims can even be far away from a blast itself and still hear an explosion, which can cause white matter abnormalities. “So when I hear [the victims] were hearing these odd sounds and there was potential exposure, and you think of advanced weapons, you think this is feasible that there’s some kind of exposure that’s going on that could have caused this.”

“ Experts remain stumped. ”

Edward Lobarinas, an associate professor at the University of Texas, Dallas, disagrees. Lobarinas’ research focuses on hearing loss, but he’s also studied the side effects of drugs on hearing. He thinks a sonic weapon would be nearly impossible to implement. For one thing, research into the weaponization of sound has historically been on two types of sound: ones where they were very obvious and very loud, like the flash grenade or sound grenades used to scatter crowds in riot; or really, really high sounds that are painful to the human ear.

“There have been long range acoustic devices, the sort of device that generates high-level sounds focused on a beam,” Lobarinas told The Daily Beast, describing the sound beams as directional like a flashlight’s beam. “It’s often used on cruise ships when they might encounter pirates. It’s typically used to cause discomfort and pain.”

But this sort of sonic weapon isn’t easy to manipulate or cheap: It requires sophisticated equipment and a lot of power to run, which means it would require an instrument that is expensive, probably visible, and makes its presence known with a directional beam—going against reports of “directional acoustic phenomena,” or the idea that a sound is only audible in certain locations.

These symptoms—a constant humming paired with neurological issues—sound familiar to Richard Salvi, a professor of communicative disorders and sciences, who specializes in the study of tinnitus (the perception of constant ringing in the ear, like the title character of the movie Baby Driver).

Tinnitus is related to hearing loss from age, a loud noise, or drugs. The brain of a person who has tinnitus is different from a normal brain, in that it displays neuroplastic changes that Salvi described as being akin to amping up a car radio as you drive away from a home station, and the sound becomes increasingly less clear. “You start losing the [clarity of the] sound, and you turn up the volume but there’s static, and turning up the volume actually makes it worse,” Salvi told The Daily Beast.

Tinnitus was, until recently, considered a dysfunction of the inner ear, but Salvi said imaging studies he and his colleagues have done from the 1990s forward show that more is going on in the brain than we might realize. “We had patients clench their jaws and make it [tinnitus] louder or quieter, and we saw that areas of the brain became active,” he said, pointing to parts of the brain related to memory and emotion in the hippocampus that became activated. That lines up with the cognitive issues that sonic attack victims are reporting and further brings into question if a sonic weapon was used.

That’s not to say that the Cuban “sonic attack” victims definitely had tinnitus; their symptoms simply line up with the condition and might give us insight into what caused their mysterious condition along with their future condition and treatment options.

“ Poisoning using an easily available pharmaceutical is a tantalizing suspect at play primarily because of how innocuous it could be. ”

One of the theories that seems to triumph over the others in understanding tinnitus and how it affects a person’s hearing and brain is that the victims were poisoned with higher-than-normal dosages for a drug that they were not meant to take, therefore accentuating their adverse reaction. Both Lobarinas and Salvi point to research on anti-cancer drugs having side effects that line up with what the “sonic attack” victims are reporting: hearing loss, tinnitus, dizziness, and nausea. Salvi said drugs prescribed to alleviate the side effects of chemotherapy, like Cisplatin, often rely on platinum in the main part of the molecule to work and have side effects that also eerily line up with what the attack victims are reporting. Research on neurotoxicity and ototoxicity—toxins that affect the brain and ear, respectively—with regard to chemotherapy suggest that hearing loss and cognition trouble is not only a normal side effect but can be severe in high doses.

In fact, poisoning using an easily available pharmaceutical is a tantalizing suspect at play primarily because of how innocuous it could be. Poisoning by food, drinking water, or even an aerosol is totally possible, Lobarinas said, saying the fact that the AP report mentions neurological damage as well as hearing loss lines up. “It has to be a neural toxin that causes neural atrophy as opposed to a person being exposed to a sound, because of the brain part,” Lobarinas said.

And that’s where the anti-cancer-drug-as-poison argument gains more traction: “the brain part.” Lobarinas was quick to say he isn’t necessarily pinpointing a specific anti-cancer drug as the culprit—“Lots of drugs have tinnitus and hearing loss and nausea and memory loss as side effects, even high doses of aspirin can do that”—but he strongly maintains the idea that a toxin can affect not only a person’s hearing but their brain too.

The AP report pointed out that white matter tracts or myelanated axons in the brain were affected, which means that the toxin had to affect not only how the brain processes sound but also the very building blocks of the body’s nervous system.

Michael Dwyer, an assistant professor of neurology and biomedical informatics at the University of Buffalo’s Department of Neurology, explains that the brain is composed of two major types of tissue: gray matter and white matter. “The gray matter is kind of like the actual computational units, the neuronal cell bodies,” he told The Daily Beast. “The white matter is the wiring closet of the brain, composed almost entirely of axons that helps connect different parts of the brain.” White matter links gray matter (which holds the locations of nerve cell bodies), acting as pathways for nerve impulses.

The white matter tracts are essentially wires, he said, and when they’re damaged, different regions of the brain are impeded from working together and coordinating. Think of it like a computer and your internet network: If the computer (the gray matter) is in good shape and fully functioning on a healthy internet connection (the white matter), all systems are good to go. If the internet connection is down, then it doesn’t matter if the computer is brand new and up to date. The breakdown of myelin from an immune system attack can affect balance, sight, and cognition, and lead to diagnoses of multiple sclerosis.

But our understanding of how hearing loss connects with white matter is fairly limited, Salvi said. Imaging technology has improved our understanding of how the two are connected, and while there probably is a relationship, consistent patterns haven’t been recognized.

“ If I were to list what is likely to have caused this attack, I'd start with a drug or some kind of toxin at the top of my list,” Lobarinas said. “Psychogenic would be second. And third—or even lower than third?—would be the use of a sound device. ”

The sneaky neural damage that has affected these patients proves that the sound exposure is potentially so low it’s hard to hear. That’s on the level of what medical journals have reported and science has shown to be repeatedly true about tinnitus and other similar anti-cancer drugs.

But anti-cancer drugs aren’t the only ones to potentially affect a person’s auditory and neural mechanisms. Salvi said aminoglycosides used in antibiotics are relatively cheap and widely available, often causing hearing loss in patients. Anti-malarial drugs—common in places like Cuba and required for government employees who travel to these locales—can cause similar side effects, particularly in high doses. And, as Lobarinas pointed out, high doses of aspirin—“like a whopping dose, just way, way, way too much,” Salvi said—can cause tinnitus that is not permanent, a key note given some patients are reporting feeling better after treatment.

Heavy metal—most commonly, mercury—exhibits similar signs of brain damage and vertigo, according to John Caravanos, a professor of environmental health sciences at New York University and an expert on mercury poisoning. What makes mercury less likely is that it’s an airborne toxin: “It’s possible through food, but most of what I see is airborne or mercury incorporated in soil,” Caravanos said. While Caravanos said mercury vapor is damaging to neurological tissue, mercury doesn’t cause hearing loss and can cause muscle twitching—a symptom none of the victims reported.

Lobarinas has studied the effects of blast injuries on animals, and he’s found evidence of both auditory and neural damage—but never down to the white matter level. “We’re talking about intense trauma from an explosion,” he said. “None of the noises we have ever used have resulted in white matter damage.”

A recent argument that has arisen is the idea that the sound these victims are hearing is psychogenic. On Monday, a panel of Cuban scientists declared the American case to be psychogenic, a mass auditory hallucination, based on the fact that those who worked close to these diplomats—in their homes, for example, or lived as neighbors—didn’t demonstrate any of these symptoms. No sonic weapons were found, and other airborne toxins, such as a common mosquito fumigant used in the area, were not found in high concentrations.

But that the victims all suffered a psychogenic episode is probably a faulty conclusion, Lobarinas insisted. Twenty-four people is a lot of people, and to enact that kind of auditory hallucination is difficult to orchestrate, he said. Salvi agreed, saying auditory hallucinations are often connected to conditions like schizophrenia, and those patients report hearing people talking or music. “But tinnitus sounds are not as elaborate, they’re more primitive and constant,” Salvi said. “It’s more like hissing, buzzing, like bacon frying in a frying pan.” Indeed, the Cuban “sonic attack” patients reported a constant hum, according to various reports.

“If I were to list what is likely to have caused this attack, I’d start with a drug or some kind of toxin at the top of my list,” Lobarinas said. “Psychogenic would be second. And third—or even lower than third?—would be the use of a sound device.”

What makes the most recent report that brain abnormalities were found even more mystifying was the fact that signs of concussions or mild traumatic brain injuries were found in patients. Injuries that devastate the brain and its structure are often the result of blasts or loud explosions—commonly found among war veterans. But the 24 victims had had no such exposure.

The symptoms the diplomats are experiencing line up with how white matter damage can affect someone’s health. Adam Brickman, an associate professor of neuropsychology at Columbia University and an expert in the AP’s report, said abnormalities within white matter can manifest themselves in the ways victims are reporting.

But Brickman emphasized that whether these victims will experience long-term effects is largely dependent on the cause of the damage and difficult to do without knowing its source—which is, of course, the whole mystery here. “You can treat that underlying disease and hopefully the symptoms would subside with the successful treatment of the cause,” Brickman said. “Without a hypothesis about what’s causing the damage, there’s not much we can do.”

While it’s unclear if these sounds are to blame for the damage, what is clear is that the damage won’t necessarily have lasting effects. Dwyer said that white matter damage is not always permanent. “There’s a lot of evidence after people have a stroke, for example, that they’re able to relearn how to do certain tasks, and in many cases that’s not necessarily because the damage has healed but because the brain has learned to reroute around that damage and work past it.”

There are physical therapists to help with the balance and special orientation, speech therapists to help with cognition, occupational therapists to help with day-to-day functioning and talk therapists to help with the emotional component. Medication may even be prescribed to help with sleep or pain issues that result from the abnormalities.

But these therapies aren’t proven to completely repair damage.

“In the brain injury world, no one’s been able to show that white matter disruption followed by therapy will result in changes to the white matter disruption,” Gore said. “But when you get a brain injury, it’s not a true statement that it never heals.”

No one can be sure how each victim will be affected, or whether all the white matter damage was in the same place in each brain with each individual experiencing the same amount of damage—or not. That lack of specificity means doctors will be tracking the victims for the remainder of their lives.