The CIA has assessed that the “majority” of reported cases of unexplained medical symptoms known as “Havana syndrome” can be “reasonably explained by medical conditions or environmental and technical factors,” a senior CIA official told ABC News.

The spy agency has assessed it’s “unlikely that a foreign actor, including Russia, is conducting a sustained, worldwide campaign harming U.S. personnel with a weapon or mechanism,” they added.

But they left the door open to the possibility that some personnel have been attacked by a still-unknown actor or device, saying a foreign actor’s role has not been ruled out “in specific cases. We’re still looking.”

The issue has vexed U.S. officials for over five years now after the first incidents were reported by personnel at the U.S. embassy in Cuba. Since then, scores of cases have been reported on nearly every continent in over a dozen countries, especially after the CIA and State Department urged employees to come forward if they experienced symptoms. But it was never clear how many of those reports were later confirmed as medically diagnosed cases.

In a rare statement, CIA Director Bill Burns said those symptoms are “real,” his agency’s commitment to providing care for officers is “unwavering” and its investigation is “not done.”

“We are pursuing this complex issue with analytic rigor, round tradecraft, and compassion and have dedicated intensive resources to this challenge,” he said. “While underlying causes may differ, our officers are suffering real symptoms.”

In a note to all staff obtained by ABC News, Secretary of State Antony Blinken also offered strong support for employees and encouraged diplomats and their families to continue reporting potential incidents.

“Those who have been affected have real stories to tell — their pain is real. There is no doubt in my mind about that,” he wrote.

But many of the affected personnel are outraged or upset by the CIA’s assessment, with some like Marc Polymeropoulos, a retired CIA agent who was affected while on assignment in Moscow, fearing they will not be believed or will be “mocked and vilified.”

“I remain grateful of the health care that Director Burns has agreed to provide for those who have been impacted, but now victims are being shamed and mocked,” Polymeropoulos said, calling it “a return to the early days of Havana where officers were not believed.”

A declassified internal government watchdog report found that the State Department moved too slowly to address the issues when personnel first reported incidents and symptoms in November 2016. Symptoms have included headaches, dizziness, cognitive difficulties, tinnitus, vertigo and trouble with seeing, hearing, or balancing. Many officials have suffered symptoms years after reporting an incident while some have been diagnosed with traumatic brain injuries.

Beyond Cuba, cases of what the Biden administration has called “anomalous health incidents,” or AHI’s, have been reported in China, Austria, Germany, Vietnam, India, Uzbekistan and Colombia, among other countries. There have even been reports of incidents in the U.S., although the White House said the vast majority are overseas.

But the CIA assessment found that “previously undiagnosed illnesses, environmental factors, reporting out of an abundance of caution” led to the majority of the cases, the senior official said.

“Many of the reports came in following growing workforce awareness of AHI’s – after requests by departments and agencies for personnel to come forward,” they added. “This finding doesn’t call into question at all the fact that our officers are reporting real symptoms and experiences. It’s just that there’s not one single cause that can be explained.”

But after media reports emerged, some critics cast doubt again on whether U.S. personnel experienced anything at all, sparking anger in other corners that the CIA had undermined its personnel and those from other agencies.

“The CIA’s interim conclusions are incredibly disappointing, insulting to those who are suffering, and highly suspect,” whistleblower attorney Mark Zaid, who represents over a dozen affected employees, said in an email to ABC News. “Once again, it is demonstrated that the failure of the government to produce a uniform, expert report only causes further controversy rather than resolution.”

Zaid said the agency was “more likely” issuing it “to allay a workforce which in recent months has been refusing overseas assignments in the wake of an overwhelming number of reported new cases among its ranks.”

“Other agencies [are] furious no coordination occurred & they disagree,” he added in a tweet, calling the interim report “disinformation.”

“It’s not disinformation. That’s absurd,” the senior CIA official said in response, adding the assessment was conducted “with the normal partners” and through “the intelligence community process.”

Blinken also tried to address personnel’s concerns about being believed. While he declined to address the assessment during a press conference in Berlin on Thursday, he told reporters employees “have had real experiences, real symptoms, and real suffering, and we are going to continue to do everything we can with all the resources we can bring to bear to understand, again, what happened, why, and who might be responsible.”

What is clear is that the report is preliminary. Lawmakers called for the CIA to continue to probe the issue, especially those cases that remain unexplained.

“Today’s assessment, while rigorously conducted, reflects only the interim work of the CIA task force,” said Sen. Mark Warner, D-Va., chair of the Senate intelligence committee, which “will continue pressing for answers on a bipartisan basis,” he added.

His Republican counterpart, ranking member Marco Rubio, R-Fla., reiterated that, saying the CIA “must continue to make this issue a priority and seek answers to the causes of mysterious symptoms, including brain injury, and whether they can be attributed to the work of a foreign government or a specific weapon or device, particularly in a core group of cases.”

It’s unclear exactly how many cases are in that “core group” that remains under active investigation. The senior CIA official said that it numbers around a “couple dozen” and that it could still involve a “foreign actor.”

“We’re not ruling it out in specific cases. We’re still looking,” they said, but added, “There are no patterns or linkages at this stage.”

Russia had long been suspected in some circles as being behind the incidents, but it’s unclear how or with what device such an attack would be possible. Blinken said last week that he and other U.S. officials have raised the issue with the Russians even without clear attribution.

Asked whether he would raise the issue with Russian Foreign Minister Sergei Lavrov when they meet Friday, Blinken declined to comment Thursday.