At least 24 diplomats and family members were affected, some with damage that they may carry for years, officials testified at a Senate Foreign Relations subcommittee hearing Tuesday.
While the incidents remain a mystery, State Department officials who testified said they believe Cuba has clearer information about who is behind what they classified as "attacks" on US diplomats working on the communist-run island.
At a news conference Tuesday night in Havana, Josefina Vidal, the director general of US affairs at the Cuban Foreign Ministry, blasted the hearing.
"There is no type of evidence," Vidal said. "There is not even evidence of what occurred, when it occurred, why it occurred, who or what was the motive. We don't even have information or evidence that the health problems originated in Cuba."
During the hearing, US officials detailed how personnel came to experience a variety of symptoms including sharp ear pain, headaches, ringing in one ear, vertigo, disorientation, attention issues and signs consistent with mild traumatic brain injury or concussion. In nearly all cases -- 24 in total -- the ailments were preceded by some sort of "acoustic element," such as a "high-pitched beam of sound" or a "baffling sensation akin to driving with the windows partially open in a car."
Todd Brown, the State Department's diplomatic security assistant director for international programs, admitted the US cannot guarantee the safety of its remaining diplomats in Havana -- reduced in number since the threat emerged -- because it still doesn't understand the nature of the incidents, and is providing guidance to staff based on what affected personnel have experienced so far.
Dr. Charles Rosenfarb, a medical director for the agency, said staff have been advised to move away from any strange sounds they hear, since the duration of exposure appears correlated to the severity of symptoms.
That advice seemed to frustrate the ranking Democrat on the subcommittee, Sen. Bob Menendez of New Jersey, who compared the directive to the hide-under-your-desk air raid drills that were supposed to protect school kids in the event of a nuclear attack in the Cold War.
"Ridiculous," he said.
All the affected US personnel have experienced some improvement over recent months, said Rosenfarb, with 10 returning to work either full time or part time.
The subcommittee's chairman, Sen. Marco Rubio, a Florida Republican, faulted the State Department for its failure to form an Accountability Review Board to review the incident and for what he called its slow response to cases reported in late 2016 and early 2017.
In his testimony, Francisco Palmieri, the acting assistant secretary of state for Western Hemisphere affairs, assured the senator that Secretary of State Rex Tillerson has now decided to form an ARB, which will convene after Congress has been formally notified.
The Accountability Review Board is an internal State Department mechanism to review security incidents involving diplomatic personnel. The most well-known recent review board followed the deadly attack on a diplomatic compound in Benghazi, Libya, in 2012.
Echoing Rubio's criticisms, Menendez cited a "bureaucratic, inadequate and troubling" response at the State Department and US Embassy over the past year.
"The failure of leadership at the department and at post, the sluggish reaction to the initial reports of afflicted personnel, the aloof response of the medical team at the State Department, silence from diplomatic security to the rest of the department is simply staggering," said Menendez.
Both Rubio and Menendez cast doubt on the Cuban government's claims of ignorance when it comes to the cause of the incidents, as well as their more recent efforts to discredit the accounts of affected diplomats.
The idea that such an attack would go unnoticed by Cuban security officials in heavily surveilled Havana, said Rubio, was "outside the realm of reasonable -- it's ridiculous."
Menendez, for his part, said it was "unfathomable" that Cuban elements were not either involved or at least aware of more than they were admitting to.
That skepticism is shared by the State Department. At a briefing Tuesday afternoon, the agency's undersecretary for public diplomacy, Steve Goldstein, said bluntly, "We believe that the Cuban government has the answer to this, and that they should be doing more to assist us in bringing this to resolution."
Goldstein also told reporters the State Department is not currently considering returning to full staffing levels at the embassy.
The Cuban government has called the alleged attacks "science fiction" and complained that Washington has not shared much in the way of medical details or let their investigators inspect the diplomatic residences where many of the incidents took place.
Tillerson told reporters at a news conference last month that the US is "convinced these were targeted attacks."
While the secretary has not accused the Cuban government of being behind the incidents, he said he holds the government responsible for allowing them to continue.
"What we've said to the Cubans is [it's] a small island, you got a sophisticated security apparatus, you probably know who's doing it, you can stop it," Tillerson said in December. "It's as simple as that."
But the inability of the US government to pinpoint the direct cause of the incidents has unsettled US lawmakers, investigators and diplomatic officials.
In his opening remarks, Brown, the State Department security official, acknowledged that, "unfortunately, this remains a perplexing case."
Rosenfarb, for his part, admitted that, "managing this evolving situation is challenging."
"Mission personnel describe a multitude of symptoms, many of which are not easily quantifiable and not easily attributable to a specific cause," he said.
The officials would not comment on a recent Associated Press report
that suggested that FBI investigators "uncovered no evidence that sound waves could have damaged the Americans' health."
However, Brown suggested that an acoustic cause was still a possibility.
"I don't know that I would rule it out entirely," he said. "The acoustic element could be used as a masking piece."
"I'm not claiming that it's acoustic," he clarified. "I just know there's been an acoustic element associated with the sensations and the feelings."
Over the weekend, Republican Sen. Jeff Flake of Arizona -- who was spotted at the hearing but did not directly question the officials -- told CNN, "The Cubans bristle at the word 'attack.' I think they are justified at doing so. The FBI has said there is no evidence of an attack. We shouldn't be using that word."
Several Canadian diplomats were also affected.