The U.S. intelligence community has concluded that a foreign country was not responsible for the so-called Havana Syndrome ailments involving U.S. officials working overseas.

This findings in a new intelligence assessment come as a disappointment to U.S. diplomats and intelligence officials who believe they suffered attacks and are still dealing with serious health problems.

The episodes were first reported by U.S. officials at the American Embassy in Havana, Cuba, in 2016. Some 1,500 cases among U.S. government staffers have now been reported worldwide. The vast majority of those cases have been resolved and were linked to causes such as existing medical conditions.

However, about two dozen current and former officials are still suffering from chronic ailments that have defied explanation, according to some of those officials who remain afflicted.

'Highly unlikely' a foreign country was responsible

Two intelligence officials familiar with the new report briefed a small number of journalists on Wednesday. The intelligence community cannot say exactly what happened in these episodes — but now believes it's pretty sure of what didn't happen in Havana and elsewhere.

Seven different U.S. intelligence agencies were involved in the investigation, and five found it was "highly unlikely" a foreign country was to blame. One said it was "unlikely," and one didn't take a position.

The officials also said there was "no credible evidence" that a foreign adversary has a weapon capable of inflicting the kind of harm suffered by the U.S. officials.

The assessment goes against what many people suspected, including many of the intelligence officers and diplomats who suffered these ailments.

NPR spoke with two of them, who remain convinced they suffered an attack, possibly with some sort of energy weapon, perhaps a microwave. But the two former officials, who requested anonymity, acknowledged that they don't have proof of what caused their ailments.

The symptoms are not the same in all the cases. But many recall the exact moment when they suffered sharp, piercing pain in their head, which caused them to be dizzy, nauseous, suffer migrane headaches, an inability to think clearly or even function.

They said they never had these problems before, and have now been plagued with them for years.

Attorney Mark Zaid, who's representing more than two dozen clients in these cases, said he's had access to some classified information and believes key information has yet to come out.

"I can say the U.S. government has a lot more information than what it is publicly revealing today. And that is where a lot of the unanswered questions arise from," said Zaid.

The two intelligence officials who gave the briefing answered reporters' questions about the assessment, but the report itself remains classified.

Medical conditions, environmental factors suspected

Reporters asked if a foreign government wasn't responsible, and no weapon or device was detected, then what caused these illnesses?

The officials said the individual cases varied, but collectively, they were probably linked to "pre-existing medical conditions, conventional illnesses and environmental factors."

The officials emphasized that the different ailments contributed to the belief there was no one single cause.

They also said that they didn't find what they were looking for — a foreign adversary who was responsible — but did learn a lot of things they weren't looking for.

For example, a faulty air conditioning or heating system can cause changes in room pressure that can cause headaches, they said.

As they investigated areas where cases were reported, they came across criminal activity, including weapons dealers and drug dealers operating nearby. But when they pursued these leads, sometimes for weeks or months, they never found any link between the criminals and the ailments suffered by the U.S. officials.

This report is the most comprehensive to date. CIA Director William Burns called it "one of the largest and most intensive investigations in the agency's history."

He also stressed that the findings "do not call into question the experiences and real health issues that U.S. government personnel and their family members have reported while serving our country."

Those afflicted are receiving medical treatment, and in some cases, have now received financial compensation under a law passed by Congress last year.