Originally Published: 01 MAR 23 12:59 ET

Updated: 01 MAR 23 16:46 ET

By Kylie Atwood and Jennifer Hansler, CNN

(CNN) -- The US intelligence community cannot link any cases of the mysterious ailment known as "Havana Syndrome" to a foreign adversary, ruling it unlikely that the unexplained illness was the result of a targeted campaign by an enemy of the US, according to a US intelligence assessment published on Wednesday.

The latest conclusion comes years after the so-called syndrome first emerged and defies a theory that it could have been the result of a targeted campaign by an enemy of the US.

The new assessment echoes an interim report from the CIA last year that found it unlikely that the "anomalous health incidents," as they are formally known, were the caused by "a sustained worldwide campaign" by Russia or any other foreign actor.

Wednesday's assessment also goes further in finding that there is no credible evidence that a foreign adversary has a weapon or collection device that is capable of causing the mysterious incidents, US intelligence officials said.

Officials also explained that the medical analysis has also evolved in a way that points away from adversarial involvement.

"I can share with you that most IC agencies have now concluded that it is 'very unlikely' a foreign adversary is responsible for the reported AHIs. IC agencies have varying confidence levels because we still have gaps given the challenges collecting on foreign adversaries -- as we do on many issues involving them," said Director of National Intelligence Avril Haines.

The mysterious illness first emerged in late 2016, when a cluster of diplomats stationed in the Cuban capital of Havana began reporting symptoms consistent with head trauma, including dizziness and extreme headaches. In subsequent years, there have been cases reported around the world, including clusters of at least 60 incidents in Bogota, Colombia and Vienna, Austria. There have been about 1,500 reported cases across the US government in 96 different counties including some cases reported this year, officials said Wednesday, but the number of reported incidents has dropped significantly in the last year.

The assessment, the result of the work of seven intelligence agencies, draws on the immense resources of the US intelligence community, including a review of hundreds of incidents and a wide range of factors surrounding them, officials explained.

No definitive answers on cause

Wednesday's assessment, however, does not provide definitive answers on or what caused the ailment that has sickened hundreds of US government personnel and family members worldwide.

There is no one explanation for these incidents. Instead, there are many different possible causes including environmental as well as social factors and preexisting medical conditions, officials said.

The assessment is likely to lead to further frustration among those impacted who have chastised the US government for not taking the condition seriously enough or slow-rolling the investigation.

"There is something counterintuitive to all of this. If doctors are diagnosing some of us with a qualified injury to the brain in the line of duty and we are not saying it was a foreign adversary, what was it from?" said one former CIA agency officer who experienced symptoms.

The intelligence community workforce was notified of the assessment on Wednesday, officials said. Sufferers were notified in recent days that the assessment was coming and some received a call from CIA Director Bill Burns, one source said.

In a statement, Burns said the assessment is "one of the largest and most intensive investigations in the Agency's history," stating that it "reflects more than two years of rigorous, painstaking collection, investigative work, and analysis by IC agencies, including CIA."

"I want to be absolutely clear: these findings do not call into question the experiences and real health issues that US Government personnel and their family members -- including CIA's own officers -- have reported while serving our country," he said.

'Extremely aggressive' investigation

The investigative efforts were "extremely aggressive" and involved "a high degree of risk," one official explained. Intelligence officers vigorously studied what happened in the hours, days and weeks surrounding the incidents, the official explained.

In some instances they found malfunctioning HVAC systems, which can cause discomfort to humans, and in other cases there were computer mice that created surprising disruptions.

"We weren't finding what we expected to find," said one of the US officials. "There is no one explanation for any of this."

There was also criminal activity that occurred around some of the incidents -- such as the presence of weapons dealers. But when intelligence officials chased those leads -- by looking into the criminals' past, their family and their travels -- they found no connection to the mysterious health incidents.

Officials even considered extraterrestrials as a cause, but found no linkage, they said.

Officials said that evidence pointed against foreign involvement, including citing "confusion" on the topic among key adversaries.

On the whole, officials did not find evidence to validate one of their incoming assumptions that one or more state actor was causing the incidents, they explained.

There is nothing to indicate that these incidents were the result of an insider attack, the officials said. The officials would not discuss if the US has a weapon that could have caused these incidents.

The findings also follow a report from a panel of experts last year -- including scientists inside and outside of government -- that found that "pulsed electromagnetic energy" emitted by an external source could have "plausibly" caused the mysterious incidents. While the latest intelligence assessment doesn't count out that possibility completely, it appears to cast doubt on it by concluding that no US adversary has the plausible weapon or mechanism that would be needed for that to happen.

US officials said that one of the lessons learned from the investigation was that the US government needs to do a better job looking after the health and safety of the workforce.

Over the last year, the CIA and the State Department began providing compensation to victims whose symptoms required at least a year of medical assistance. Compensation efforts are "separate and distinct" from the intelligence assessment and will continue to be implemented, officials said.

Following the release of the assessment, administration officials were clear that support would continue.

"We are going to continue to see to it that our colleagues who report these incidents are treated with respect and compassion, receive timely access to medical care and we'll continue to process Havana Act payments based on the eligibility criteria that's been spelled out in the law," State Department spokesperson Ned Price said.

However, the future of the US government's probe into these incidents is a bit murky.

Officials would not definitively say if the intelligence community's task force devoted to this effort would stay up and running, but Haines said that the work "will and must endure." The Pentagon also has a team of experts that continues to investigate the matter.

This story has been updated with additional information.