Another diplomat working at the United States Embassy in Havana has been sickened by a mysterious attack, bringing to 25 the number of personnel who have fallen ill there, the State Department said on Thursday.

The case was the first confirmed in Cuba since August and could suggest that whatever caused the illnesses in late 2016 and the first part of 2017 had started again or was continuing. Complicating the situation for officials was news this spring that at least one diplomat at a consulate in Guangzhou, China, had experienced symptoms almost identical to those reported by diplomats in Havana.

Officials have frankly admitted in background conversations that they still have no idea who or what may be behind the attacks, but a growing number of committees and organizations are looking into the problem, including an interagency working group led by the deputy secretary of state, John J. Sullivan, with representatives from the Department of Health and Human Services and the Justice Department.

“The interagency community continues to work diligently to determine the cause of the symptoms, as well as develop mitigation measures,” Heather Nauert, the State Department spokeswoman, said at an afternoon news briefing.