Secretary of State Antony Blinken told U.S. diplomats in a message Thursday that the government still does not know how to stop the "Havana Syndrome."
Blinken said in the email, which was obtained by NBC News, that the U.S. was doing all it could to investigate unexplained health incidents.
Blinken said he'd been hearing "growing concerns" about the incidents, which he said date to "at least 2016."
"Employees going abroad are anxious about whether they or their families are at risk," Blinken wrote. "That's completely understandable, and I wish we had more answers for you."
Read more at NBCNews.com.