WASHINGTON (AP) — The State Department confirmed Thursday that another U.S. diplomat has been affected by mysterious health incidents in Cuba, bringing the total of Americans suffering from such ailments to 26.

Spokeswoman Heather Nauert said the diplomat was “medically confirmed” to have experienced health effects similar to those reported by other members of the U.S. Havana diplomatic community.

This and another case confirmed last week resulted from a single occurrence in late May in a diplomatic residence in which both officers were present, Nauert said. They were the first confirmed cases in Havana since August 2017.

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She said U.S. government agencies continue “to work diligently to determine the cause of the symptoms, as well as develop mitigation measures.” The confirmed Cuba patients have been found to have a range of symptoms and diagnoses including mild traumatic brain injury, also known as concussions.

Nauert says Cuba has assured the U.S. it will continue its investigation. It was informed of the “attack” on May 29, she said.

The United States has said that the Cuba incidents started in late 2016 but has not said what caused them or who was behind them.

Cuba has adamantly denied involvement or knowledge. Initial speculation centered on some type of sonic attack owing to strange sounds heard by those affected, but an interim FBI report in January found no evidence that sound waves could have caused the damage, The Associated Press has reported.

“Whoever uses the term ‘attack’ to refer to symptoms reported by U.S. diplomats is consciously lying, with a well-defined political agenda,” Cuba’s director-general of relations with the U.S., Carlos de Cossio, wrote on Twitter. “The United States has a long history of resorting to political manipulation and aggression by overt and covert methods to achieve its political and economic goals.”

One American government worker at a U.S. consulate in China who also confirmed to have been affected, raising concerns that the incidents occurring in Cuba have spread.