A sudden sensation of pressure in the head. An unfamiliar sound that seems to come from nowhere. Dizziness, headaches and difficulty concentrating that persist long after the initial experience.
Since 2016, some US diplomats, intelligence officers and other government personnel have reported symptoms like these in incidents that became known as Havana syndrome. The reports triggered medical investigations, intelligence assessments and speculation about whether a foreign government had developed a weapon capable of affecting people without leaving obvious physical evidence.
Nearly a decade later, the central question remains difficult: what caused these experiences, and could an external technology explain at least some of them?
The story has become more complicated as new reporting has revisited intelligence investigations and a suspected device capable of producing pulsed radio-frequency energy. But a device’s existence, its technical capabilities and its possible connection to a reported illness are three different questions.
The key distinction is that the symptoms reported by affected personnel are real experiences, while the cause of the incidents has not been conclusively established. Understanding what investigators have found—and what remains unproven—is essential to assessing the Havana syndrome mystery.
What is Havana syndrome?
The term Havana syndrome emerged after personnel at the US Embassy in Havana, Cuba, began reporting unusual health incidents in late 2016. Similar reports later came from personnel and family members in other locations.
US government agencies increasingly use the term anomalous health incidents, or AHIs, because the reports cover a range of symptoms and circumstances rather than one clearly established disease.
Reported symptoms have included headaches, dizziness, balance difficulties, nausea, tinnitus, hearing changes, head pressure, visual disturbances and problems with concentration or memory. Some individuals described experiencing an unusual sound or sensation shortly before their symptoms began.
The severity and duration have varied. Some people reported persistent difficulties that affected their work and daily lives, while others experienced symptoms that differed in character and course.
These reports prompted concern because some incidents appeared sudden and, in certain accounts, were associated with a particular location or sensory experience.
However, the term Havana syndrome should not be mistaken for proof of a single biological mechanism. A collection of similar complaints does not necessarily mean that every case has the same cause.
The US Department of Defense continues to provide information and medical resources for people reporting AHIs. Its official AHI information page describes the reported symptoms and the government’s ongoing clinical response.
The suspected weapon theory
One of the most controversial explanations is that some incidents resulted from exposure to directed energy, particularly pulsed radio-frequency energy.
Radio-frequency energy is part of the electromagnetic spectrum used in communications, radar and many other technologies. Devices can produce radio-frequency signals in different ways, and their effects depend on factors such as frequency, power, pulse characteristics, distance and exposure conditions.
The possibility that particular forms of electromagnetic energy could affect biological systems has therefore attracted scientific and security interest. But that broad technical fact does not establish that a weapon caused Havana syndrome.
In 2026, renewed media attention focused on reporting about a portable device acquired by a US government investigative agency and examined for its potential to produce pulsed radio-frequency energy. Reporting also raised questions about components and possible Russian connections.
These developments have intensified debate about whether investigators adequately examined every potential external explanation.
Nevertheless, several distinctions remain essential. A device capable of emitting radio-frequency energy is not automatically a weapon designed to cause neurological symptoms. The presence of components associated with a particular country does not independently establish who built or operated the device. And finding that a technology exists does not demonstrate that it was used during a specific reported incident.
To establish a causal connection, investigators would need reliable evidence linking a device to an incident, showing that the relevant exposure occurred and demonstrating that the exposure could account for the symptoms observed.
Public reporting about a suspected device has not, by itself, conclusively established that it caused Havana syndrome.
What did intelligence investigations conclude?
US intelligence assessments have played a central role in the controversy.
In March 2023, the US Intelligence Community released an updated assessment of anomalous health incidents. The assessment reported that the agencies involved considered it very unlikely that a foreign adversary was responsible for the incidents as a whole.
Investigators cited gaps in the evidence connecting the reported symptoms to a foreign actor, as well as medical and technical findings that did not support several assumptions behind earlier suspicions.
The assessment did not mean that affected personnel had invented their symptoms. Nor did it establish one universal medical explanation for every report.
Instead, the intelligence community concluded that the available evidence did not support attributing the broader pattern of incidents to a foreign adversary.
That distinction became increasingly important as journalists and former officials continued to question whether particular cases might differ from the larger group.
Subsequent media reporting in 2026 has renewed scrutiny of the investigations and the interpretation of evidence concerning potential foreign involvement. Such reporting is relevant to understanding the debate, but it should not be treated as equivalent to a publicly verified finding that a foreign state caused the illnesses.
The Office of the Director of National Intelligence’s 2023 statement explains the assessment and its conclusions.
The broader lesson is that intelligence judgments can be challenged or reassessed when new evidence emerges. But a change in public debate is not, by itself, proof that an earlier conclusion was false.
What have medical studies discovered?
Medical research has investigated whether people reporting AHIs show consistent patterns of brain injury or measurable biological changes.
In March 2024, researchers at the US National Institutes of Health published findings from detailed clinical assessments and advanced brain imaging studies involving federal employees who reported anomalous health incidents.
The researchers found no significant evidence of MRI-detectable brain injury or consistent differences in most of the clinical measures examined compared with control participants.
The findings were important, but they require careful interpretation.
An absence of detectable abnormalities on particular scans does not mean that a person’s symptoms are imaginary. Medical conditions can produce substantial difficulties without a clear structural abnormality on conventional imaging. The results also cannot exclude every possible mechanism, especially one that might not leave a persistent or detectable marker using the methods available.
The NIH researchers identified functional neurological disorders or significant somatic symptoms in a substantial proportion of participants. These are recognised clinical phenomena, not synonyms for pretending to be ill. They involve genuine symptoms and require appropriate clinical assessment.
The study did not establish that all reported incidents shared a psychological cause. Nor did it prove that a directed-energy exposure had occurred.
Instead, it showed that the particular imaging and clinical measures used did not reveal a consistent pattern of detectable brain injury that could explain the cases as a group.
The original findings are available in the National Institutes of Health announcement.
Why the mystery remains unresolved
One challenge is that Havana syndrome is not a single, uniformly documented event. Reports emerged in different places, at different times and under different circumstances.
Some individuals described sudden sensory experiences. Others reported symptoms without the same type of preceding event. Medical evaluations also produced varied findings, making it difficult to establish one mechanism that explains every case.
Researchers must distinguish between at least three questions:
First, what symptoms did individuals experience, and how did those symptoms affect them?
Second, what biological or environmental mechanisms could plausibly produce those symptoms?
Third, is there evidence that a particular external actor or device was responsible for any of the incidents?
Evidence that answers one question does not automatically answer the others.
A patient may have genuine and disabling symptoms even when the cause is unknown. A proposed physical mechanism may be scientifically possible without having been demonstrated in a particular case. And an intelligence investigation may find insufficient evidence of foreign involvement without resolving every medical question.
These distinctions help explain why the controversy continues despite years of research.
What evidence would settle the debate?
A convincing explanation would require evidence that connects the reported experiences to a specific cause.
For an alleged directed-energy incident, this could include reliable measurements of exposure, verified technical information about a suspected device, a credible chain of evidence connecting the device to a particular location and time, and biological findings consistent with the proposed mechanism.
Medical investigations would also need to consider alternative explanations and differences between individual cases.
For other hypotheses, researchers would similarly need testable predictions and reproducible findings. A theory should become more convincing when independent evidence supports it, not simply because it explains an unanswered question.
The investigation also has a human dimension. People reporting these symptoms need appropriate medical care, clear communication and a serious assessment of their concerns regardless of whether the eventual cause proves to be neurological, environmental, psychological or something else.
The US Government Accountability Office has documented challenges in how the Department of Defense communicates with affected personnel and coordinates access to care. Its 2024 report on Havana syndrome discusses these treatment and administrative issues.
The Havana syndrome mystery is not yet solved
The Havana syndrome story sits at the intersection of medicine, national security and scientific uncertainty. It includes reports of genuine health problems, investigations into possible external threats and competing interpretations of evidence that remains incomplete.
The possibility of a novel device deserves rigorous examination if credible evidence supports it. But technical capability, suggestive reporting and geopolitical suspicion cannot substitute for proof that a device caused a particular illness.
Likewise, medical findings that fail to identify a consistent injury should not be misrepresented as evidence that affected people are not suffering.
The most defensible conclusion is that the cause of Havana syndrome has not been conclusively established. The 2023 intelligence assessment did not find convincing evidence that a foreign adversary was responsible for the incidents as a whole, while later reporting has renewed questions about particular technologies and cases.
Those questions warrant evidence-led investigation rather than premature certainty.
The mystery may eventually become clearer through improved medical research, access to reliable technical evidence and careful reassessment of individual incidents. Until then, the responsible approach is to take patients seriously, scrutinise new claims and distinguish what has been demonstrated from what remains possible but unproven.
