Microwave strikes may be responsible for the mystery sonic illness caused by strange noises that led the United States to evacuate several Americans from China and Cuba in the last year, The New York Times has reported.

Douglas H Smith, Director of the Centre for Brain Injury and Repair at the University of Pennsylvania, was a part of a medical team that examined 21 diplomats who left Cuba. Smith is the lead author of a report on this mystery illness published in the Journal of American Medical Association.

The diplomats and their families reported traumatic brain injuries and permanent hearing loss. Some experienced nausea, headaches and ear-ringing, and some now suffer from problems with concentration and word recall.

Microwaves were considered the main suspect and the medical team was increasingly positive the diplomats had suffered brain injury, Smith told the newspaper. “Everybody was relatively sceptical at first,” he said. “And everyone now agrees there’s something there.”

Microwaves are electromagnetic waves with a wavelength shorter than average radio waves but longer than infrared radiation. They are used in ovens, radars, phone communication, among other everyday appliances. But concentrated beams of microwaves can enable sharper focus, similar to a dish antenna. The size of the human head makes it a good antenna for picking up microwave signals, the report said, citing a study published in the US National Center for Biotechnology Information.

However, the team did not mention the microwaves in its report published in March, describing the origin of trauma as a highly directional “unknown energy source”. The diplomats seemed to have developed signs of concussion without having received any blows to the head.

Strikes with microwaves are more likely to be the cause of painful sounds and traumas than other factors such as sonic attacks, viral infections, and anxiety, The New York Times quoted unnamed experts as saying. This phenomenon is known as the Frey Effect, named after American scientist Allan H Frey, who discovered that microwaves can trick the brain into believing what seem to be ordinary sounds. These false sensations might explain most of symptoms displayed by the diplomats, including including ringing, buzzing and grinding.

Frey, 83, has been questioned by government officials. He has also said that microwave radiation may have been used to attack the diplomats.

While the Federal Bureau of Investigation refused to comment, the US State Department said the investigation was yet to identify the cause or the source of the illness.