Most cases of so-called “Havana Syndrome” can be explained by environmental causes, undiagnosed medical conditions or stress, rather than the actions of a foreign power, The New York Times reported on Thursday, citing officials of the Central Intelligence Agency (CIA).

Most of the 1,000 cases reviewed by US researchers are explainable and show that the mysterious ailment is unlikely to have been caused by Russia or another foreign adversary, CIA officials told the newspaper, describing interim results of a study.

The agency is still investigating two dozen unexplained cases that may offer clues as to whether a foreign power is behind the condition that affected US diplomats, officials and family members in places including Vienna, Paris, Geneva and Havana, the newspaper said, citing CIA officials.

In addition to those cases, there are a significant number that remain unexplained, the Times quoted a CIA official as saying. The agency did not immediately respond to a request for comment from Reuters on Thursday.

The condition first came to public attention in 2016, after dozens of diplomats from the US embassy in Cuba complained of illness. Symptoms included migraines, nausea, memory lapses, and dizziness.

CIA Director William Burns said in a statement to the Times that the agency is investigating a complex matter with “analytical rigor, strong modern techniques and compassion,” noting that agency officials had experienced real symptoms.

“Although we have reached some important interim conclusions, we are not done,” Burns said in the statement. “We will continue our mission to investigate these incidents and provide access to world-class care to those who need it.”

Earlier this month, Secretary of State Antony Blinken said in an interview that the United States still did not know what Havana Syndrome was or who was responsible.

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