WASHINGTON — The C.I.A. has found that most cases of the mysterious ailments known as Havana syndrome are unlikely to have been caused by Russia or another foreign adversary, agency officials said, a conclusion that angered victims.

A majority of the 1,000 cases reported to the government can be explained by environmental causes, undiagnosed medical conditions or stress, rather than a sustained global campaign by a foreign power, C.I.A. officials said, describing the interim findings of a comprehensive study.

The C.I.A. is continuing its investigation into two dozen cases that remain unexplained. Those cases, said a U.S. official briefed on the findings, offer the greatest chance of yielding clues to whether a foreign power is responsible for some of the unexplained health incidents that have plagued American diplomats and C.I.A. personnel in Havana and Vienna, among other cities.

In addition to those two dozen cases, a significant number of others remain unexplained, the official said.

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