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WASHINGTON (AP) — A panel of intelligence experts hasn’t identified a single culprit for apparent brain injuries reported by U.S. personnel that have been linked to so-called “Havana syndrome,” but several potential causes remain plausible, including the use of devices that emit beams of directed energy, officials said Wednesday.

The CIA announced last week that the agency considers it unlikely Russia or another foreign adversary is mounting a broad campaign to attack Americans with energy-emitting devices. While most cases have been linked to other causes by doctors and experts, there remains a smaller subset of several dozen cases that experts believe could be explained by the deliberate use of energy.

In examining the science behind the incidents, the panel’s work is the latest announcement on a sensitive issue in diplomatic and scientific circles. U.S. officials and lawmakers have emphasized they regard the reported illnesses seriously and that any deliberate attack on U.S. personnel would be met with a firm response. But intelligence agencies have not made public evidence that an adversary is to blame. The uncertainty about the cause of the illnesses has added to friction between officials and those suffering from symptoms.

Psychological factors alone cannot explain the characteristics displayed by people in that smaller number of cases, said officials who briefed reporters Wednesday on condition of anonymity under ground rules set by the Office of the Director of National Intelligence.

The use of pulsed electromagnetic energy “plausibly explains” those characteristics in some cases, as does the use of ultrasound waves at close range, officials said. Experts so far have not identified a specific device that could have been used to target American personnel in the field.

The officials who briefed reporters Wednesday said there were significant gaps in what the government knows. Among the recommendations the panel has made is standardizing the information collected across U.S. agencies and creating new markers for identifying and caring for what the government calls “anomalous health incidents.”

The U.S. is also looking for ways to protect officers and prevent future cases. While officials who briefed reporters would not specify what protection measures they have recommended, they urged any employee who believes they have been affected to come forward immediately.

“While we don’t have the specific mechanism for each case, what we do know is if you report quickly and promptly get medical care, most people are getting well,” one official said.

In a statement, Director of National Intelligence Avril Haines and CIA Director William Burns said the U.S. government “remains committed to providing access to care for those who need it, and we will continue to share as much information as possible with our workforce and the American public as our efforts continue.”

“Havana syndrome” cases date to a series of reported brain injuries in 2016 at the U.S. Embassy in Cuba. Incidents have been reported by diplomats, intelligence officers and military personnel in the Washington area and at global postings.

The Biden administration has faced pressure from lawmakers in both parties to investigate cases linked to “Havana syndrome” and provide better care for people who have reported the sudden onset of sometimes debilitating headaches, dizziness, and other symptoms.

President Joe Biden on Tuesday named a senior National Security Council official to coordinate the government’s response to possible incidents related to Havana syndrome.