WASHINGTON • Something mysterious and disturbing has happened to US State Department personnel, first in Cuba and now in China.
Strange high-pitched sounds - "buzzing", "piercing squeals", "grinding metal", as the Cuba staffers later told doctors - preceded an eruption of health problems, including headaches, dizziness, confusion, ear pain, hearing loss, insomnia and fatigue.
Last year, then-Secretary of State Rex Tillerson referred to what happened in Havana as "attacks".
Numerous news stories and opinion columns speculated about "sonic attacks" using some kind of unknown acoustic weapon.
The mystery spread this year to China: A staff member in Guangzhou experienced "subtle and vague, but abnormal sensations of sound and pressure", in the words of the State Department.
The department has issued a health alert to staff in China, telling anyone who experiences "any unusual acute auditory or sensory phenomena accompanied by unusual sounds or piercing noises, do not attempt to locate their source".
What we know about sonic weapons
WHAT IS A SONIC WEAPON OR ATTACK?
Studies have shown that prolonged exposure to ultrasonic sound can result in hearing loss and human tissue damage. It would take a device about a size of a matchbox to produce noise that could, at close range, induce feelings of anxiety or difficulty concentrating. But high frequency sound does not travel well through any kind of barrier, like a wall or even a curtain.
HOW WOULD IT WORK?
The frequency of the sound could be high (ultrasound) or low (infrasound). Ultrasound frequencies above 20,000Hz or 20kHz are inaudible to humans but can damage parts of the ear, says the BBC.
Infrasound or lower frequencies below 20Hz are used by animals to communicate. If very loud, it could lead to vertigo or uncontrollable bowel movement. It could also cause vomiting.
An attack using infrasound would need to use a large array of subwoofers and so would not be very covert, New Scientist says.
WHAT DAMAGE CAN A SONIC ATTACK CAUSE?
Dizziness, headache, hearing loss, vomiting, vertigo and brain damage could be some of the symptoms of an acoustic attack.
WHO IS BEHIND THE ATTACK?
FBI agents probing the incidents have been unable to find the cause. With no hard evidence, former US officials have entertained theories that involve a third country, like Russia or North Korea, perpetrating the attacks.
HAS SOUND BEEN USED AS A WEAPON EARLIER?
Yes, sound has been used as a non-lethal, yet potentially harmful, weapon, says CNN.
In several countries, including Britain, it has been used to prevent young people from loitering. Long-range acoustic devices have been used by the authorities to control crowds.
The US army has used heavy metal to make Iraqi prisoners of war cooperate in interrogations, and deprive them of sleep, the BBC says.
NYTIMES, WASHINGTON POST, REUTERS
What the phenomenon lacks so far is any solid information about cause and effect. It is not even clear that the ailments suffered have an external origin.
Officials have produced no evidence that anyone has intentionally attacked the Americans, nor is there any obvious environmental cause. And there is no sign of a rogue virus or other pathogen.
No one disputes that American employees of the department have been suffering from a long list of symptoms. But a research report about the Cuba staffers, published on Feb 15 as "Preliminary Communication" in the prestigious Journal of the American Medical Association (Jama), has triggered scepticism among outside researchers.
The report came from the University of Pennsylvania Medical School's Centre for Brain Injury and Repair, where 21 State Department personnel from Cuba have been examined and treated.
Most had persistent symptoms, such as cognitive problems, sleep impairment and hearing dysfunction. All but three had reported hearing unusual sounds in their homes or hotel rooms before the onset of their symptoms.
In Philadelphia, doctors concluded that the patients "appeared to have sustained injury to widespread brain networks without an associated history of head trauma".
Their report offered no explanation for the injuries, however.
The doctors detected no clear physical origin in the brain: Eighteen of the 21 patients showed nothing unusual on a brain scan, and the other three had "mild" or "moderate" damage to white matter that the investigators acknowledged could be due to pre-existing disease processes.
Under the heading of "Meaning", the paper notes: "The unique circumstances of these patients and the consistency of the clinical manifestations raised concern for a novel mechanism of a possible acquired brain injury from a directional exposure of undetermined etiology."
But the paper left many outside experts perplexed.
Two neuroscientists at the University of Edinburgh, Dr Robert McIntosh and Dr Sergio Della Sala, published a criticism last month, saying the evidence of pathological mental impairment in the patients was "almost unbelievably flimsy".
In e-mails to The Washington Post last week, Dr Della Sala wrote: "Whatever the cause of the symptoms, the Jama paper presented evidence far too thin to support the existence of brain damage."
Another University of Edinburgh neurologist, Dr Jon Stone, said via e-mail that the data published by Jama "is compatible with individuals who have a genuine functional disorder consisting of dizziness, headache and cognitive symptoms, which we see routinely in neurological practice. These disorders... are so common that it would not be hard to find a cluster of cases in a larger group of at-risk individuals".
The cases in Cuba began in November 2016 and were first reported to embassy officials there at the end of that year.
They continued until last August, then reportedly stopped. Those 10 months were a period of political turmoil and a chilling of US-Cuba diplomatic relations after the election of President Donald Trump.
Cuban scientists looked into the cases after conferring with US officials. The Cuban conclusion: The issue was a "collective psychogenic disorder".
According to a story in the journal Science, the US government gave Cuban investigators recordings of irritating sounds around the residences of embassy personnel, which turned out to most closely match the buzzing of Jamaican field crickets.