Experts are growing more and more suspicious that microwave weapons were responsible for inflicting brain damage on nearly 40 U.S. diplomats and their families in Cuba and possibly China, the New York Times reports.
The Americans reported strange high-pitched sounds outside their Havana homes and hotels in 2016 and 2017, followed by severe headaches, nausea, fatigue, dizziness and hearing loss.
A report on the injuries earlier this year did not mention a cause.
But after examining more than 20 victims, the report's author, Dr. Douglas Smith, head of the brain injury center at the University of Pennsylvania, tells VOA a microwave weapon looks more and more like the culprit.
"What kind of mechanical forces can change the brain in the way that's ultimately very similar to a head blow, but there is no evidence of having that very large mechanical injury," he asked.
Dr. Smith said he and his team examined the victims at the request of the State Department and all had the same symptoms as if they suffered concussions.
Smith brushed off suggestions that this may have been some kind of mass hysteria among the diplomats, saying they were examined separately and that no one can fake such injuries as balance and cognition problems.
Allan Frey, a U.S. scientist, discovered in 1960 that the human brain can perceive microwaves as sounds.
According to the New York Times, the United States had researched using microwaves as weapons, including ways to beam speech into an enemy's head and possibly paralyze him.
U.S. intelligence warned more than 40 years ago that the Soviet Union was also looking into microwave weapons.
The Times also reported that a Washington lawyer obtained a National Security Agency statement saying that a foreign power built a weapon "designed to bathe a target's living quarters in microwaves, causing numerous physical effects, including a damaged nervous system."
The precise cause of the sonic attacks on the Americans is still unknown, including who was behind them and why.
President Donald Trump has directly blamed Cuba and, and last year expelled 15 Cuban diplomats from Washington.
Cuba denies any involvement and calls Trump's move political.
The U.S. also brought home a number of Americans from the consulate in Guangzhou, China, in June after diplomats there showed the same symptoms of a sonic attack. A Chinese spokesman said officials investigated and found nothing suspicious.