According to six people briefed on the matter, the CIA has ruled out the possibility that the mysterious symptoms known as Havana Syndrome are the result of a sustained global campaign by a hostile power aimed at hundreds of US diplomats and spies.

The agency cannot rule out foreign involvement in about two dozen cases, including many of those that began in 2016 at the US Embassy in Havana. Another category of cases is classified as unresolved. According to the sources, the agency has found plausible alternative explanations in hundreds of other cases of possible symptoms.

According to the sources, the notion that widespread brain injury symptoms were caused by Russia or another foreign power targeting Americans around the world, either to harm them or to gather intelligence, is unfounded.

According to the sources, people who have experienced possible Havana Syndrome symptoms and have been briefed on the assessment have expressed deep disappointment. Some have noted that the CIA’s findings are considered preliminary and were not coordinated with other agencies, including the Defense Department.

“The CIA’s newly issued report may be labeled ‘interim,’ and it may leave open the door for some alternative explanation in some cases, but to scores of dedicated public servants, their families, and their colleagues, it has a ring of finality and repudiation,” a group representing US officials who have reported suspected incidents said in a statement.

Nonetheless, almost no one who has been closely following the issue is surprised that many recent cases of what the Biden administration refers to as “anomalous health incidents” have plausible explanations unrelated to possible hostile acts. When government agencies began urging any employee who had experienced unexplained headaches and dizziness to report it, it was inevitable that many of the cases would not fit the parameters of what doctors had discovered in the diplomats and spies who had suffered brain injuries.

Officials said there is an intensive intelligence gathering and analysis effort underway to resolve those cases.

Beginning in late 2016, US diplomats and spies in Cuba began reporting strange sounds and sensations, followed by unexplained illnesses and symptoms such as hearing and vision loss, memory and balance issues, headaches, and nausea.

According to a 2018 report, US intelligence officials considered Russia a leading suspect in what some of them believed to be deliberate attacks on diplomats and CIA officers abroad. However, in the three years since then, the spy agencies have not uncovered sufficient evidence to identify the cause or perpetrator of the health incidents.

In a 2020 report, the National Academies of Sciences, Engineering, and Medicine stated that some of the observed brain injuries were consistent with the effects of directed microwave energy, which Russia has long studied.

According to the National Academies of Sciences report, a team of medical and scientific experts who studied the symptoms of up to 40 State Department and other government employees concluded that nothing like them had previously been documented in medical literature. Many people reported hearing a loud sound and feeling pressure in their heads, followed by dizziness, unsteady gait, and visual disturbances. Many suffered long-standing debilitating effects.

The CIA report aims to address a growing concern as more cases are reported: that a US adversary has managed to harm hundreds of Americans in dozens of countries while evading detection by the US.

The interim report, according to US officials, is not a final conclusion of the Biden administration or the entire intelligence community. The White House National Security Council has formed a task force comprised of outside medical and scientific experts to investigate Havana Syndrome, with results expected in the near future, according to officials. The Defense Department, the FBI, and the State Department are all still investigating the syndrome’s origins and ways to mitigate it.