WASHINGTON — Probably the most possible reason for a series of mysterious afflictions that sickened American spies and diplomats overseas previously a number of years was radiofrequency vitality, a kind of radiation that features microwaves, the Nationwide Academies of Sciences, Engineering, and Medication has concluded in a report.

The conclusion by a committee of 19 specialists in medication and different fields cited “directed, pulsed radiofrequency vitality” as “essentially the most believable mechanism” to elucidate the sickness, which got here to be often known as Havana syndrome, although they mentioned that they may not rule out different doable causes and that secondary elements might have contributed to signs, in response to a replica of the report obtained by The New York Instances.

The report, which was commissioned by the State Division, supplies essentially the most definitive clarification but of the unusual sickness that struck scores of presidency staff, first on the U.S. Embassy in Havana in 2016, after which in China and different nations. Lots of the officers suffered from dizziness, fatigue, complications, and lack of listening to, reminiscence and stability, and a few had been compelled into everlasting retirement.

C.I.A. officers visiting abroad stations additionally skilled related signs, The Instances and GQ journal reported in October. The officers had been touring to debate countering Russia covert operations with overseas intelligence businesses, a undeniable fact that provides to suspicions that Moscow is behind the episodes.

Although couched in cautious, scientific language, the brand new report reveals sturdy proof that the incidents had been the results of a malicious assault. It attributes the sicknesses to “directed” and “pulsed” — quite than “steady” — vitality, implying that the victims’ publicity was focused and never the results of extra widespread sources of microwave vitality, similar to, for instance, a cellphone.

It additionally mentioned the committee discovered the rapid signs that sufferers reported — together with unusual sensations of ache, strain and sound that usually appeared to emanate from a specific route, or occurred in a particular spot in a room — had been extra in line with a directed “assault” of radiofrequency vitality.

The committee thought-about different causes, like chemical exposures and infectious ailments, however mentioned they appeared unlikely.

The report mentioned that the variability of the incidents, which appeared to have an effect on completely different individuals in several methods, left open the doable affect of “psychological and social elements.” And it mentioned that a few of the victims could also be experiencing a situation referred to as “persistent postural-perceptual dizziness,” a nervous system dysfunction that produces a protracted feeling of vertigo or unsteadiness.

The episodes have been the topic of a lot hypothesis and controversy. Lots of the victims, in addition to some authorities officers and outdoors scientists, have long argued that radiofrequency vitality was the most probably trigger, probably the results of a weapon wielded by a overseas energy.

However since 2018, the U.S. authorities has declined to take a position publicly on the instances, and a few scientists have promoted alternate theories, like a form of psychological illness that unfold within the nerve-racking setting of overseas missions.

Amid the controversy and confusion, a few of the stricken officers have complained that the USA has didn’t help them. In a number of instances, the federal government initially refused to grant go away and supply the required medical care, the officers mentioned. And with the federal government silent on the potential for a overseas assault, lots of the victims had been left feeling that the general public believed that they had made all of it up.

A number of of the victims have accused Secretary of State Mike Pompeo and different Trump administration officers of downplaying the problem in an try and keep away from disrupting worldwide ties. They now ask how President-elect Joseph R. Biden Jr. and his nominee for secretary of state, Antony J. Blinken, will reply, particularly given the brand new scientific findings.

The State Division gave the report back to some congressional officers and others on Thursday and Friday and informed them to not share it, after lawmakers had pressured the company for months to launch the report. The Instances and NBC Information individually obtained the report on Friday, and NBC earlier reported the findings.

”We’re happy this report is now out and might add to the info and analyses that will assist us come to an eventual conclusion as to what transpired,” the State Division mentioned in an announcement on Saturday.

The division additionally mentioned that “every doable trigger stays speculative” and that numerous elements, together with the committee’s lack of entry to some info due to potential safety considerations, “restrict the scope of the report,” although “they don’t reduce its worth.”

For the Trump administration, acknowledging that the incidents had been the results of a overseas assault might have necessitated evacuating American missions in China, disrupting an necessary financial relationship. The administration did take a tougher method in Cuba, which aligned with its bigger aim of reversing President Barack Obama’s diplomatic opening with Havana.

The query of Moscow’s doable culpability is a thorny one, given the sensitivities round President Trump on any issues involving Russia or President Vladimir V. Putin. Moscow has denied any position, and Gina Haspel, the C.I.A. director, has not concluded the Kremlin was accountable. However some C.I.A. analysts who’re Russia specialists, diplomats and scientists contend that proof factors to Moscow, which has a protracted historical past of experimenting with the expertise.

The report doesn’t level to a perpetrator, although it mentions “important analysis in Russia/U.S.S.R.” on pulsed radiofrequency expertise, in addition to the publicity of navy personnel in Eurasian communist nations to microwave radiation. The Soviet Union bombarded the American Embassy in Moscow with microwaves within the Seventies and ’80s. In a 2014 document, the Nationwide Safety Company mentioned a microwave weapon utilized by a hostile nation, which individuals conversant in the doc mentioned was Russia.

Mark Lenzi, a diplomatic safety officer who was stricken with the signs whereas working in Guangzhou, China, in early 2018, mentioned that the administration’s therapy of its staff, together with its efforts to “deny and canopy up inconvenient scientific and medical details,” had left him angrier at his personal authorities than the federal government that injured him.

“My authorities appeared the opposite means after they knew I and my household had been injured,” he mentioned. “This report is just the start and when the American individuals know the total extent of this administration’s cover-up of the radiofrequency assaults in China particularly they are going to be outraged.”

Mr. Lenzi has sued the State Division for incapacity discrimination. The Workplace of Particular Counsel has been pursuing two investigations into the State Division’s conduct.

The report by the Nationwide Academies additionally comprises a stark warning about the potential for future incidents, and the U.S. authorities’s capability to detect them, or to mount a response. The truth that American authorities staff reported afflictions not solely in Cuba and China but additionally in Russia and different nations raises questions on how widespread the incidents could also be.

The committee was not able to evaluate “particular eventualities involving malevolent actors,” Dr. David Relman, a Stanford College professor who led the committee, wrote in a preface to the report. But, he mentioned, “the mere consideration of such a state of affairs raises grave considerations a couple of world with disinhibited malevolent actors and new instruments for inflicting hurt to others.”

The report recommends that the State Division act now to ascertain plans and protocols so it may instantly start an investigation if related incidents happen sooner or later.

“The bigger concern is preparedness for brand new and unknown threats that may compromise the well being and security of U.S. diplomats serving overseas,” the report concludes. “The subsequent occasion could also be much more dispersed in time and place, and much more troublesome to acknowledge rapidly.”

The panel mentioned its findings had been hampered by the federal government’s gradual and uneven response to the incidents, through which completely different sufferers had been evaluated by numerous strategies and clinicians at completely different factors of their sickness. It additionally mentioned that the data made out there on sufferers from China was “too sparse and fragmentary to have the ability to draw any substantive conclusions about these instances,” and due to this fact the report focuses on occasions surrounding the U.S. Embassy in Havana.

The scientists despatched the report back to the State Division in August, and company officers there then put it below assessment. Lawmakers pressed the division to publicly disclose the findings, saying its failure to launch the data match with a sample of secrecy and inaction by the Trump administration. In interviews in October, Dr. Relman criticized the division for not appearing sooner to launch the report.

Some victims mentioned on the time that the Trump administration was attempting to keep away from addressing its shortfalls towards the protection of U.S. authorities staff abroad, particularly forward of the November elections. Requested in late October by a reporter in regards to the sicknesses, Mr. Pompeo didn’t point out the report and mentioned solely that the federal government was unable to determine the cause.

A number of lawmakers have forcefully pressed the State Division to be extra accountable and supply correct well being and work compensation advantages to the entire victims and affected relations. Senator Jeanne Shaheen, Democrat of New Hampshire, has inserted a provision on long-term advantages into the most recent protection funds invoice that Congress is predicted to go this month, although Mr. Trump has threatened to veto the measure for causes unrelated to the availability.

“Their sicknesses and struggling are actual and demand a response from Congress,” Ms. Shaheen mentioned. “Whereas I’m inspired by the progress we’re seeing, way more have to be finished to uncover the supply of those incidents and be certain that no different public servant suffers on this means.”

Julian E. Barnes contributed reporting.