US officials still stumped on mystery illnesses in Cuba, open door to 'viral' or 'ultrasound' cause A new report claimed the FBI uncovered "no evidence" of a "sonic attack."

 -- State Department officials told Congress Tuesday they would not rule out "an acoustic element" in what the U.S. says were attacks on two dozen American diplomats in Cuba, despite a new FBI report that doubts a “sonic attack” is responsible.

The different views come as the U.S. government remains utterly stumped about what happened to 24 of its personnel at its embassy in Havana who have reported symptoms similar to mild traumatic brain injury. Officials now say “a range of things” could be responsible, including “viral” or “ultrasound” attacks.

On Monday, the Associated Press reported that an interim FBI report from the bureau's Operational Technology Division said its probe "has uncovered no evidence that sound waves could have damaged Americans' health."

Todd Brown, Diplomatic Security Assistant Director for International Programs at the State Department, would not comment on the FBI report Tuesday in testimony before the Senate Foreign Relations Committee, but said he "wouldn't rule out an acoustic element entirely."

"I'm not claiming that it's acoustic. I just know there's been an acoustic element associated with the sensations and the feelings," Brown told the committee. "Whether if the FBI has determined that is not the case -- which I have not seen this report, and I don't think it's been released publicly -- that doesn't mean an acoustic element couldn't be part of another type of style of attack here."

Brown said technical experts are looking at a "range of things" that could be associated with the alleged attacks, including "viral" and "ultrasound."

The Cuban government has denied any involvement in the Americans’ symptoms and even rejected the idea that there have been attacks.

A State Department official told ABC News on Monday that they "do not have definitive answers on the source or cause of the attacks," as the investigation is ongoing. Separately, a government official familiar with the investigation told ABC News that the FBI has not been able to determine what caused the injuries to U.S. personnel.

The FBI has made four trips to Havana, Cuba and is preparing for a fifth, after interviewing 37 people and examining more than 80 employees and family members from the U.S. mission.

What is currently known

All 24 Americans have been medically confirmed to have symptoms consistent with what looks like a mild traumatic brain injury, according to Dr. Charles Rosenfarb, Medical Director for the Bureau of Medical Services at the State Department.

As ABC News previously reported, those symptoms included sharp, localized ear pain, dull unilateral headache, ringing in one ear, vertigo, visual focusing issues, disorientation, nausea, and extreme fatigue, according to Rosenfarb.

While some symptoms went away in days or weeks, other individuals reported persistent problems, including difficulty concentrating, recurring headaches, hearing loss, sleep disturbance, and imbalance while walking, Rosenfarb said.

A senior State Deptartment official confirmed that point, telling ABC News that as of now, most of the 24 Americans are better, but some have “permanent” damage. illnesses.

The first cases occurred as early as November 2016 and, in the beginning, attacks appeared in “clusters,” with several incidents occurring within a matter of days. From late March to late April of 2017, they occurred more sporadically before appearing to stop. After a period of with attacks, there were two additional incidents were reported in close proximity in late August, which were medically confirmed in late September, according to Francisco Palmieri, Acting Assistant Secretary with the State Department's Bureau of Western Hemisphere Affairs.

In response to the alleged attacks, the U.S. first approached Cuba in mid-February to demand protection for its staff. Cuba denied involvement and began its own investigation. In May 2016, After the alleged attacks continued, the U.S. expelled two Cuban diplomats, followed by nearly two-thirds of the Cuban embassy in October.

Guaranteeing Americans' safety in Cuba

Brown told Congress that the safety of American diplomats in Cuba could not be guaranteed, echoing what Secretary of State Rex Tillerson told the Associated Press last week.

“I’d be intentionally putting [U.S. diplomats] back in harm’s way. Why in the world would I do that when I have no means whatsoever to protect them?” Tillerson said. “I will push back on anybody who wants to force me to do that.”

In mid-April, Tillerson granted “no fault curtailment,” meaning employees could leave if they felt unsafe. But on Sept. 29, he ordered the departure of non-emergency personnel from the U.S. mission in Havana. Essential personnel still remain, but the State Department will be forced to decide in the coming months whether to shutter the embassy or return employees.

Brown said that there are "teams in place that can respond" to questions from those who remain in Cuba, as well as explain "how to report those types of incidents."

"Certainly not knowing what's causing it, or who's behind it, or how it's being done, it gives us very little in terms of mitigation," he said.

Since August, there have been no reported incidents.

The Accountability Review Board

Sen. Marco Rubio, R-Fla., questioned the officials about why an Accountability Review Board was not established after the State Department knew by May that "several" American diplomats had suffered a serious injury.

Rubio said that, by law, the State Department would have 120 days to establish that board after learning of the injuries, but failed to do so.

Palmieri responded that information known at the time was later contradicted and that the government had not been able to identify who perpetrated the attacks. He said it was only after late August -- after another round of attacks -- that it became "apparent" the State Department should establish an Accountability Review Board.

Tillerson ordered for that board to be convened on Dec. 11, and Palmieri said Congress should receive notification of the board soon.

What the Cuban government knew

Palmieri told the committee that he found it beyond belief that the Cuban government was not aware of the attacks.

"It's incomprehensible to us that they are not aware of how and who is responsible, and that they cannot take steps to prevents these kind of attacks from ever happening again," he said.

Palmieri acknowledged a "long history and pattern of Cuban harassment of U.S. diplomats stationed in Havana," adding it was "entirely possible" the government could have escalated their harassment to cause these attacks.

"In whatever case, they are responsible for the safety and security of U.S. diplomats stationed in Havana under the Vienna Convention," Palmieri said. "And they have failed to live up to that responsibility."

Both Rubio and Sen. Robert Menendez, D-N.J., questioned the officials about the possible involvement of Russia.

Palmieri would not say publicly whether the State Department had raised the alleged attacks against its personnel in Cuba with the Russian government.

When asked by Rubio about similar documented attacks against Americans during the Cold War, Rosenfarb acknowledged there were cases in the 1950s and 60s in which microwave beams were used against Americans at the U.S. Embassy in Moscow.

"The Cuban government either did this, or they know who did this," Rubio concluded.