NBC News reported in 2018 that U.S. intelligence officials considered Russia a leading suspect in what some of them assess to have been deliberate attacks on diplomats and CIA officers overseas. But there was not — and is not now — conclusive intelligence pointing in that direction, multiple officials who have been briefed on the matter said.

A team of medical and scientific experts who studied the symptoms of as many as 40 State Department and other government employees concluded that nothing like them had previously been documented in medical literature, according to the National Academies of Sciences report. Many reported hearing a loud sound and feeling pressure in their heads, and then experienced dizziness, unsteady gait and visual disturbances. Many suffered longstanding, debilitating effects.

“The committee felt that many of the distinctive and acute signs, symptoms and observations reported by (government) employees are consistent with the effects of directed, pulsed radio frequency (RF) energy,” the report says. “Studies published in the open literature more than a half-century ago and over the subsequent decades by Western and Soviet sources provide circumstantial support for this possible mechanism.”