The mystery ailment that has afflicted U.S. embassy staff and CIA officers off and on over the last four years in Cuba, China, Russia and other countries appears to have been caused by high-power microwaves, according to a report released by the National Academies. A committee of 19 experts in medicine and other fields concluded that directed, pulsed radiofrequency energy is the “most plausible mechanism” to explain the illness, dubbed Havana syndrome.

The report doesn’t clear up who targeted the embassies or why they were targeted. But the technology behind the suspected weapons is well understood and dates back to the Cold War arms race between the U.S. and the Soviet Union. High-power microwave weapons are generally designed to disable electronic equipment. But as the Havana syndrome reports show, these pulses of energy can harm people, as well.

As an electrical and computer engineer who designs and builds sources of high-power microwaves, I have spent decades studying the physics of these sources, including work with the U.S. Department of Defense. Directed energy microwave weapons convert energy from a power source —a wall plug in a lab or the engine on a military vehicle—into radiated electromagnetic energy and focus it on a target. The directed high-power microwaves damage equipment, particularly electronics, without killing nearby people.