The effects of microwave radiation on humans have long been the focus of weapons research in the US and elsewhere. At some frequencies, microwaves can be used to cause great discomfort—including a burning sensation—without causing long-term effects. But in others, microwaves can penetrate deeper into the body and cause symptoms that include auditory hallucinations induced directly in the brain. Evidence now suggests that strange symptoms experienced by US embassy staff in Havana and China may have been the result of attacks with a microwave—and Russian agents are now the most likely suspects behind the attacks. But skepticism about whether microwaves are to blame remains.

Last March, the Journal of the American Medical Association published details of examinations of 21 of the victims of the mysterious symptoms, finding they had "sustained injury to widespread brain networks without an associated history of head trauma." Earlier this month, the head of the team that conducted the study told The New York Times that microwaves were the most likely cause of the brain injuries. The Times' William Broad reported that a number of experts have now connected the symptoms experienced by the victims with the Frey effect, also known as the microwave auditory effect (MAE)—in which microwaves induce the sensation of sounds (or even speech) inside a person's head.

That effect, first described by American neuroscientist Allan Frey in 1961, has been the focus of repeated research by the Soviet Union, the Russian Federation, and the US. US Navy-funded research in 2003 and 2004 by WaveBand—a company later acquired by Sierra Nevada—looked into the use of MAE as a crowd control weapon called MEDUSA (Mob Excess Deterrent Using Silent Audio):

MAE results in a strong sound sensation in the human head when it is irradiated with specifically selected microwave pulses of low energy. Through the combination of pulse parameters and pulse power, it is possible to raise the auditory sensation to the “discomfort” level, deterring personnel from entering a protected perimeter or, if necessary, temporarily incapacitating particular individuals.

The research was dropped after WaveBand's acquisition. Kenneth Foster, a bioengineering professor at the University of Pennsylvania and an early researcher into MAE, told IEEE Spectrum in 2008 that the system wouldn't work as intended—”Any kind of exposure you could give to someone that wouldn’t burn them to a crisp would produce a sound too weak to have any effect." Other researchers agreed that heat would get to people before the sound did.

The US military continued to look into that particular feature of microwaves. The US Army deployed the Active Denial System, a microwave-based heat weapon, to Afghanistan in 2010 but apparently never used it. The Defense Department has moved on to research in laser-based auditory weapons for non-lethal purposes.

But other countries have apparently continued research into such weapons, without concern about the long-term physical effects a microwave weapon would have on its targets—or perhaps because of them. In 2012, Russian Defense Minister Anatoli Serdjukov announced that "directed energy weapons" and "psychotronic weapons"—weapons intended to attack the central nervous system of human targets—were part of Russia's ten-year plan for military weapons procurement.

The National Security Agency confirmed to attorney Mark Zaid in a 2012 memorandum that there was intelligence in the late 1990s that a foreign government had developed a high-energy microwave weapon "designed to bathe a target's living quarters in microwaves, causing numerous physical effects, including a damaged nervous system."

Now, US officials are certain that the symptoms experienced by victims in Havana and China are the result of deliberate attacks, and Russia is the primary suspect. According to a report from NBC News, the victims included State Department diplomatic corps members, CIA officers, at least one member of the US military, and employees of other US government agencies. The Canadian government has also reported that one of their diplomats also suffered hearing loss from a similar incident in Cuba.

Update: The Washington Post reports skepticism about microwaves being the source of the symptoms among doctors and scientists, including some doctors who were critical of the initial JAMA report. University of Cincinnati neurologist Alberto J. Espay told the Post, “Microwave weapons is the closest equivalent in science to fake news.”