Multiyear investigation by US agencies casts doubt on speculation that a hostile power was behind unexplained ailments.

The declassified findings were released on Wednesday, showing the results of an investigation conducted by seven US intelligence agencies over the course of several years.

The story was first reported by the Washington Post, which wrote that the investigation “contradicts in nearly every respect” claims from the afflicted individuals that they were “victims of a deliberate attack — possibly at the hands of Russia or another adversarial government”.

Some had speculated that “Havana Syndrome” symptoms, which include migraines, nausea, memory lapses and dizziness, could have been caused by a mysterious energy weapon, a theory that Wednesday’s report casts doubt on.

The cause of “Havana Syndrome” remains a mystery. The investigation explored 1,000 reported cases, but in January, a CIA official stated that the majority could be “reasonably explained by medical conditions or environmental and technical factors, including previously undiagnosed illnesses”, according to the news agency Reuters.

The first known cases appeared in Havana, Cuba, in 2016, with later incidents reported in countries such as Australia, Austria, China, Colombia, Germany and Russia. The US government has labelled the cases “anomalous health incidents”.

Of the seven agencies involved, the Washington Post reported that some expressed differing levels of certainty that a foreign power was not involved in the incidents, including one abstention.

However, the consensus did not differ significantly, and none of the agencies challenged the conclusion that a foreign power did not cause the symptoms.

Uncertainty about the mysterious illnesses set off widespread speculation in the US, with some officials characterising them as a targeted attack on US personnel overseas.

In October 2021, the US Congress passed a bill (PDF) called the Helping American Victims Afflicted by Neurological Attacks (HAVANA) Act, allowing government agencies to “compensate affected current employees, former employees and their dependents for qualifying injuries to the brain”.

In a statement on Wednesday, CIA Director William Burns said that the agency would work to address the “real health issues” experienced by agency personnel with “honesty and compassion”.