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Pentagon asks staff to report any mysterious symptoms of discomfort

“We will find out,” he added.

There are various reasons why the United States has struggled to identify who, and what, is responsible for the episodes. Officials have estimated that the intelligence services of several countries could be involved, each with different motives and equipment causing the illnesses, some U.S. officials say.

Officials stress that the possibility of multiple adversaries remains only a theory and intelligence officials have yet to draw firm conclusions.

But Cold War-era surveillance technology developed by the Soviet Union has proliferated in other countries, each of which has equipment that can cause symptoms similar to previous attacks.

In at least some of the Havana Syndrome cases, the technology has been used by GRU, Russia’s military intelligence agency, to listen, but not to deliberately injure, some US officials say. But other later episodes look more like deliberate attacks by the GRU, those officials said.

In other cases, Russian intelligence may not be involved at all, beyond sharing the technology. Some intelligence agencies may have used faulty or improperly calibrated microwave surveillance technology and inadvertently injured the U.S. officials they were spying on.

The first batch of cases occurred at the U.S. Embassy in Havana in 2016 and 2017. The next batch occurred at diplomatic outposts in China, a country where it would have been difficult for intelligence services. Russians to operate, according to intelligence officials.

Since then, authorities have followed cases in Europe and Asia.

Representative Adam B. Schiff, Democrat of California and chairman of the House Intelligence Committee, said in an interview this summer that several countries could be involved and that it was essential not to dismiss various explanations.