WASHINGTON — For months, the C.I.A. and government scientists have been working to find a cause of the chronic ailments reported by intelligence officers and diplomats — but the health incidents, known as the Havana syndrome, remain as mysterious now as they were a year ago.

Intelligence officials have not found any hard evidence that points to a cause. There are no intelligence intercepts implicating an adversarial spy service. No one has detected microwaves, other readings of energy pulses or any other weapons that could be to blame.

Some officials say they remain convinced Russia is involved. And the C.I.A. director, William J. Burns, delivered a warning during his trip to Moscow this month: If Russia was found to be responsible, there would be consequences.

The trouble developing evidence shows the difficulty of the problem, and suggests that absent a big breakthrough — evidence of someone using a device or an informant telling the C.I.A. about what is afoot — getting answers will be a slow, frustrating and potentially contentious process, especially for those who have been afflicted.

Some outside experts have raised the possibility that the symptoms — chronic headache, vertigo, nausea and others — are a kind of psychosomatic reaction to stress, a so-called “functional illness” — a suggestion rejected by victims and many government officials.

Some scientists believe sensory discomfort, like the strange sounds, heat or pressure associated with Havana syndrome cases, coupled with anxiety, can trigger real symptoms and sickness.

There have now been 750 official reports of possible anomalous health incidents, according to people briefed on the cases, but about three-quarters are no longer being investigated as likely cases of Havana syndrome. Some reports lacked the required sensory experience, such as heat, pressure or sound, before the symptoms’ onset, and others were found to have separate environmental or medical explanations. Of those cases, it is possible some may turn out to be psychosomatic, according to people briefed on the intelligence.

But of the approximately 200 cases of mysterious incidents still under active examination, the Biden administration does not think they were caused by functional illness or other psychosomatic reactions. In those cases, a U.S. official said, multiple explanations remain possible, including directed energy, sonic devices or other medical explanations.

Directed energy, such as microwaves, remains one of the theories, perhaps the leading theory, according to American officials. But, so far, the C.I.A. has been unable to collect hard evidence to show that any of the people suffering from symptoms of Havana syndrome have been hit with some sort of energy pulse.

Across the government, agencies are searching for clues they may have missed that could help unravel the mystery, according to officials familiar with the efforts. The examination, including the F.B.I., N.S.A. and C.I.A., involves reviewing forensic evidence, including surveillance tapes from American embassies. The government is also putting measures in place to detect any directed energy aimed at American diplomats and spies abroad.

One official said the work showed that the various agencies were determined to get to the bottom of what is happening. But, the official cautioned, the work could take time. The government needed to find “the right answer” not “the easy answer., said the official, who spoke on the condition of anonymity because much of the government’s work to determine a cause of the incidents is classified.

American officials have repeatedly said the symptoms people are suffering from are real and the C.I.A., the Biden administration and Congress have taken steps to improve access to medical care and improve compensation for victims who can no longer work.

“What I know, having talked to dozens and dozens of my colleagues who have been victimized, is that real harm is being done to real people,” Mr. Burns told a congressional hearing last month.

In a report last year, the National Academy of Sciences concluded a microwave weapon, a form of pulsed directed energy, was the most likely cause. Recent studies have indicated that directed energy or microwaves could cause brain injuries and symptoms like those of Havana syndrome.

Others briefed on the intelligence said the lack of evidence is baffling, since the kinds of directed energy known to cause injury ought to be detectable. The absence of proof could mean an adversarial power is using a technology that is unknown, and undetectable, to the United States.

It could also mean that the theory that directed energy is being used is wrong.

Some officials inside the government are convinced Russia is responsible for at least some of the incidents, and has deliberately targeted American military personnel and C.I.A. operatives. In a trip this month to Moscow, Mr. Burns told Russian officials that any sort of operation that caused severe brain injuries for U.S. personnel was out of bounds for Russia’s spy services, and that there would be consequences if Russia is shown to be responsible, according to people briefed on the conversations. The Washington Post earlier reported Mr. Burns’s warning to Russian officials.

It was the second time senior American officials have raised the issue with their Russian counterparts, who have consistently denied involvement. President Biden raised the issue with President Vladimir V. Putin of Russia in their Geneva summit earlier this year.

American intelligence agencies have also not been able to intercept any communications about rival foreign services using a device on U.S. diplomats or spies. Getting an informant that could shed light on what foreign intelligence agencies or intercepting communications of rival spies is a top priority for intelligence agencies, according to people briefed on the inquiry.

What is more, medical personnel working with the intelligence agencies have also not been able to develop a concrete diagnosis or any kind of test that can determine who has been a victim and who has other kinds of medical ailments.

Individual victims have said their doctors have found blood markers consistent with concussions and brain scans showing injury or nerve damage. But investigators looking at the broad group of victims who have reported symptoms have not been able to find a pattern, like a diagnostic test result the victims have in common.

Mark Lenzi, a State Department official who was injured Guangzhou, China, said cognitive tests in many people with Havana syndrome show similar deficits, particularly in exercises involving three dimensional shape rotations. Such tests are given to fighter pilots, and both the Army and Navy tested Mr. Lenzi earlier in his career, results that can be compared to more recent evaluations.

The government. Mr. Lenzi also said, has readings that show the presence of dangerous levels of microwave energy in China. In an unclassified workers’ compensation report he filed with the Labor Department, Mr. Lenzi recounted how his neighbor in China used a commercial detector to record high levels of microwave energy in the apartment next to his.

But follow-up tests by the government used a classified device widely known not to be as reliable at detecting directed energy, said Mr. Lenzi, whose work involves countering foreign eavesdropping including by using directed energy. When the government says directed energy is a theory but there is no evidence, “That is simply not true,” Mr. Lenzi said

“They have readings, especially in Guangzhou,” he said.

To lead the effort to find a cause of the incidents, and improve the medical care for those hurt by them, the C.I.A. has formed the Global Health Incident Cell, a group that has been reviewing all of the reports.

A senior U.S. official said the intelligence agencies “want a breakthrough because we want to know the thing or things that is producing harm.” Finding an answer, whatever it is, could not only help the government stop what is causing them but also help doctors treat the ailments.

Some former government officials say the episodes stretch back decades. Listening devices used by the Russian government in the 1990s and aimed at C.I.A. officers working in the U.S. embassy are believed by some to have caused nausea and other symptoms.

But the most recent spate of incidents began in late 2016 in Cuba, where 40 C.I.A. officers and diplomats said they heard strange noises, then reported headaches and nausea in episodes through May 2018. Those exposed the longest have reported chronic disabilities.

Since then American diplomats have been injured in Guangzhou and other cities in China. More than two dozen American officials have reported symptoms in Vienna. There have been other reports around the world involving military officers, C.I.A. personnel and diplomats.