State Department spokeswoman Heather Nauert told reporters another US diplomatic employee in Havana, Cuba has suffered "health effects" consistent with those previously experienced by employees at the embassy there, following comprehensive medical evaluation.
This is the first confirmed case since August, Nauert said Thursday.
It is the latest reported case of a mystery illness that has plagued embassy employees in Cuba, and at a US diplomatic post in China, that has in the past been described as a "sonic attack."
Nauert emphasized that the US still hasn't pinpointed the cause of the attacks or the perpetrators.
"We still don't know, to this day, what is causing it and who is responsible," said Nauert, who noted that investigations are underway in Havana, as well as Guangzhou, China, where an employee experienced similar symptoms recently.
A second employee in Cuba is being evaluated, Nauert said. The number of affected Americans in Havana now stands at 25, she said.
In the past, some officials have characterized the incidents at the embassy as "sonic attacks" or "acoustic attacks" because they have often coincided with a high-pitched sound. US officials have detailed how personnel in Cuba came to experience a variety of symptoms including sharp ear pain, headaches, ringing in one ear, vertigo, disorientation, attention issues and signs consistent with mild traumatic brain injury or concussion.
CNN reported in May that a number of diplomatic personnel in China have been sent back to the United States for further health screenings after concerns about mysterious acoustic incidents similar to those in Cuba.
Secretary of State Mike Pompeo announced in June the formation of a task force in response to "unexplained health incidents" affecting US diplomats and their family members.
A study earlier this year outlined the extensiveness of the problem, but the State Department has not pointed to a specific cause behind the mysterious incidents.