This week, Cuba’s new post-Castro president, Miguel Díaz-Canel, arrived in New York to address the UN General Assembly. Making his debut on the world stage, Díaz-Canel repudiated the “failed” US embargo against Cuba. Soon, the UN will issue its annual, and overwhelming, condemnation of the embargo—as it has done every fall for the past 27 years. Last year, 191 nations voted to denounce the bloqueo, as the Cubans call it. Only two—the United States and Israel—voted to support it.

But while the embargo remains a well-known obstacle to normal economic relations, diplomatic relations, which were fully normalized during the Obama administration, now face a far more amorphous challenge: the still-unexplained health problems suffered by more than two dozen US covert operatives and diplomatic officials in Havana between November 2016 and August 2017.

A year ago, the State Department virtually shuttered the US embassy in Havana—and expelled the majority of Cuban personnel from Cuba’s embassy in Washington—in response to the rash of ailments reported by embassy personnel, which included hearing loss, headaches, dizziness, insomnia, and cognitive impediments. The dramatic downsizing of staff—from over 50 embassy employees to around 15—was accompanied by a dire warning to US travelers to steer clear of Cuba, leading to a significant falloff of tourism to the island.

The State Department recently rescinded the travel alert, but not before the nascent private sector in Cuba, which caters to US tourism, suffered significant economic losses. Cuban entrepreneurs, as well as thousands of other Cubans with family in the United States, have been hit hard by the closure of the consulate section of the embassy, which has prevented them from obtaining visas in Havana to travel to the United States on business or to visit relatives. They are now forced to bear the expense of traveling to countries such as Mexico, Colombia, or Guyana to apply for a visa.

Moreover, as Cuba transitions into the post-Castro era, “the staff reduction at the embassy has made it more difficult for U.S. diplomats to cover significant economic and political developments in Cuba,” according to a recent impact assessment by the Congressional Research Service. “With recent developments such as a new Cuban president, new regulations on Cuba’s private sector, and the Cuban government embarking on a process to update the country’s constitution,” the CRS study advised, “U.S. policymakers may benefit from a fully staffed embassy with the ability to analyze these and other ongoing developments in Cuba.”

But the Trump administration appears to have no intention of re-staffing the embassy and restoring normal diplomatic relations until the mystery of the health problems is resolved. For more than a year, multiple US agencies, among them the CIA, the FBI, the Centers for Disease Control, and an elite military science advisory team known as the JASON Group, have undertaken major inquiries into the origins of these mysterious maladies—with no discernible conclusions. Doctors who have examined the victims have referred to their brain injuries as “the immaculate concussion.” “There is no known cause, no known individual or group believed to be responsible at this time,” a State Department spokesperson conceded earlier this month. “They have a few theories,” says one Capitol Hill staffer who has received classified briefings on the ongoing investigations, but they “are no closer to knowing how this happened or who was responsible then they were over a year ago.”

The initial theory was that the embassy personnel fell victim to some sort of acoustic aggression—a “sonic attack,” as the mainstream media summed up the phenomenon after victims reported hearing grinding chirps, or high-pitched buzzing and squealing sounds, along with a sensation of pressure and pulsating vibrations in their ears. The allegation of an “attack” derives from the commonality of the initial group of those affected: Most were either members of the CIA station in Havana or US diplomats living in residencies previously used by intelligence operatives. The CIA believed its operatives were being targeted, either by the Cuban Intelligence Directorate or by a foreign intelligence service operating in Havana, and, as The Nation has previously reported, recalled them in the summer or early fall of last year.

But investigators have dismissed the theory of a “sonic” weapon. Acoustic specialists as well as medical experts have pointed out that the symptoms could not be caused by noise. “We think the audible sound was a consequence of the exposure, because audible sound is not known to cause brain injury,” said Dr. Douglas Smith, director of the Center for Brain Injury and Repair at the University of Pennsylvania, who co-authored a controversial study in the Journal of the American Medical Association on the US personnel stricken in Cuba. In a still-classified report earlier this year, FBI investigators formally dismissed the possibility that a “sonic attack” caused the health problems.

Armed with a tape recorder, at least one US official was able to capture the sounds associated with the injuries. The recording, leaked to the Associated Press last October, underwent a rigorous acoustical analysis by a team of computer scientists and engineers at the University of Michigan and Zhejiang University. They provided a second theory focused on ultrasound: The metallic grinding noises could be the result of an accidental collision of ultrasound waves emanating from separate devices such as eavesdropping and jamming technologies. “If ultrasound played a role in harming diplomats in Cuba,” their study states, “then a plausible cause is intermodulation distortion between ultrasonic signals that unintentionally synthesize audible tones. In other words, acoustic interference without malicious intent to cause harm could have led to the audible sensations in Cuba.”

It’s unclear whether the scientists and doctors involved in the investigations have tested this theory to see if such a “collision” of ultrasound frequencies might result in the victims’ medical symptoms. Instead, a far more sinister theory is now generating much media attention—that a secret microwave gun, supposedly developed by the Russians, was pointed at US officials in Cuba, and perhaps in China as well, where at least one US consulate officer was stricken with similar symptoms this summer. On September 1, The New York Times ran a story, “Microwave Weapons Are Prime Suspect in Ills of U.S. Embassy Workers,” which provided an overview of the history of efforts by both the US military and the Russians to build radio-frequency weapons that could “invisibly beam painfully loud booms and even spoken words into people’s heads. The aims,” according to the article, “were to disable attackers and wage psychological warfare.” The article speculated, but provided zero evidence, that the injuries suffered in Cuba were the result of such weapons.

NBC News soon followed up with a report that US intelligence agencies had intercepted communications indicating that the Russians were behind the “attacks” in Cuba. According to NBC sources, “the evidence is not yet conclusive enough, however, for the U.S. to formally assign blame to Moscow.” In a variant of this theory, on September 7 The Miami Herald reported that a team of neuro-technology experts working with the State Department believed that the Havana victims had been hit by “directed energy weapons”—electronic devices using ultrasound or electromagnetic pulses “which can cause injury by creating ‘cavitation,’ or air pockets, in fluids near the inner ear.”

A number of doctors have already cast doubts on the microwave-weapon theory. “Microwave weapons [are] the closest equivalent in science to fake news,” University of Cincinnati neurologist Alberto Espay told The Washington Post. His opinion is shared by a team of Cuban neuroscientists who traveled to Washington earlier this month, at the invitation of the State Department, to meet with US officials and doctors familiar with these cases. At a press conference at the Cuban embassy, Dr. Mitchell Joseph Valdés-Sosa pointed out that the level of microwaves needed to injure the brain would likely turn it into mush. “You’d have to practically vaporize the person before microwave can damage the brain,” he told NBC News.

The Cuban team articulated another theory, supported by a number of doctors from around the world, that the ailments are the result of what Valdés-Sosa calls “psychogenic factors” and a contagious anxiety syndrome among the embassy corps. There was no doubt that the US personnel were sickened, he emphasized, but, he argued, psychogenic disease—physical ailments that derive from emotional or mental stresses—needs to be seriously considered as a potential cause.

With CIA operatives, Russians, and ultra-secret ultrasound, microwave, and electromagnetic weaponry alleged to be involved, it is no surprise that a dark cloud of mystery overshadows this diplomatic impasse in US-Cuban relations. And as the impasse enters its second year, notes Emily Mendrala, who directs the Center for Democracy in the Americas, “the ambiguity of the situation is being used by those who want to undermine policies of engagement between the US and Cuba,” which have advanced US business, cultural, and political interests in Cuba’s socioeconomic transition. Concerned that the bilateral gains made during the Obama era are being sabotaged, Cuba has called on the United States to restore normal functions of both the US embassy in Havana and the Cuban embassy in Washington. But the State Department maintains that there will be no return to “business as usual” until Cuba takes unspecified steps to guarantee the security of US personnel on the island.

“The new US government has devoted itself to artificially fabricate under false pretexts, scenarios of tension and hostility that serve nobody’s interests,” President Díaz-Canel asserted in his speech to the UN General Assembly—an apparent allusion to the embassy health issues. But he also reiterated that Cuba “stands ready to develop respectful and civilized relations with the US government on the basis of sovereign equality and mutual respect.” The Trump administration, as he told reporters with a degree of pessimism that is undoubtedly shared at the UN, is “an administration with which it is difficult to form an equal relationship.”

Editor’s note: This article was updated on October 1 with a new penultimate paragraph.