WASHINGTON — New research into a recording of a mysterious sound heard by U.S. embassy workers who fell ill in Cuba indicates that the high-pitched sound matches the chirp of a certain species of cricket.
Starting in late 2016, American diplomats and intelligence officers started reporting strange symptoms that developed after they experienced unexplained sounds and sensations in their homes in Havana.
Twenty-six Americans were eventually deemed by the U.S. to have been affected by what the government has called "specific attacks" on its diplomats leading to hearing, vision and cognitive problems.
Numerous recordings were made by workers who heard strange sounds in Cuba, and in October 2017, The Associated Press obtained and published one such recording, describing it as a "high-pitched whine" that "sounds sort of like a mass of crickets."
Now a pair of biologists has studied the recording and say that it closely matches the calling song of the Indies short-tailed cricket, found in parts of the Caribbean but not known to exist in Cuba.
The study by researchers Alexander Stubbs of the University of California Berkeley and Fernando Montealegre-Z of the U.K.'s University of Lincoln adds to the ongoing mystery about what happened to the U.S. embassy workers, but does not answer the central question: how they ended up with conditions that the State Department says includes hearing loss, memory issues and mild traumatic brain injury.
"It focuses narrowly on what was heard on the recording," Stubbs tells NBC News. After the initial cases in Cuba were detected, U.S. diplomats posted there were encouraged to record strange sounds that they heard and in some cases were given recording devices by the government.
Last year, an interim FBI report said that sound waves alone couldn't have caused the damage, raising the possibility that whatever sound the diplomats heard was not the direct cause of their injuries. Some investigators have suggested that the sound could have been the byproduct of something else that caused the damage or beamed at the diplomats as a decoy to obscure another type of technology.
"The fact that the sound on the recording was produced by a Caribbean cricket does not rule out the possibility that embassy personnel were victims of another form of attack," Stubbs and Montealegre-Z wrote in their paper, which has not yet been peer-reviewed or published in a scientific journal.
Crickets, when they produce sound with their wings, create a "distinct acoustic signature" that can be measured for several characteristics, the researchers wrote — similar to how each person's fingerprint is unique. Using technology that creates visual representations of sound signals, they were able to study characteristics such as pulse repetition rate, oscillations per pulse and duration.
"The way that they produce their sound, the wing decelerates through each sound pulse, which allows you to see the frequency change through each sound pulse," Stubbs said in an interview.
Cuba has adamantly denied any knowledge of or involvement in any attacks in U.S. diplomats. In 2017, Cuban experts who had studied recordings of the sound provided to Cuba by the U.S. government said they'd determined they were very similar to the sounds of cicadas and crickets that live along Cuba's coast.
The Indies short-tailed cricket, known scientifically as anurogryllys celerinictus, has been found previously in Jamaica, Grand Cayman and the Florida Keys, but isn't known to exist in Cuba. Still, a similar type of cricket is found in Cuba, and the researchers hypothesize that it's possible that the Indies short-tailed cricket is present there, too.
At first, the researchers studying the AP recording found that the pulse structure was "unlike any natural insect source." But they suspected that was because the diplomats had recorded the sound while indoors, where the recording might picked up echoes from the walls and floor, as opposed to recorded outside "in the field."
To test that theory, they took a recording of the Indies short-tailed cricket and played it indoors on a loudspeaker. They then recorded it again and found that it now matched the pulse structure seen in the recording from Cuba.
Previous studies of the recordings, which have also been extensively studied by the U.S. military, have come to different conclusions. Last year, engineers at the University of Michigan who studied the same AP recording in a lab determined that the sound could have been the unintended side effect of poorly built ultrasonic eavesdropping devices that combined to create audible sound heard by the diplomats.