The United States stood behind its assertion that U.S. personnel in Cuba were deliberately attacked and raised the possibility Tuesday that it was by a virus, as lawmakers and even the FBI challenged the government's initial theory of "sonic attacks."

The lack of answers more than a year after the incidents started has emboldened Cuba's defenders to argue the U.S. can't be certain anyone was harmed intentionally — especially since no proof has been publicly presented. But top State Department officials testifying before the Senate Foreign Relations Committee said even if it wasn't a sonic attack, they were sure it was an attack of some other kind.

Fueling renewed skepticism was a new FBI report, revealed by The Associated Press Monday, saying the U.S. has found no evidence sonic waves were used to harm Americans in Havana. Officials told Congress there are many theories that haven't been ruled out — including the possibility of a virus deployed intentionally to infect the workers.

Todd Brown, assistant director for the State Department's Diplomatic Security service, did not offer any evidence for why a virus might be to blame. Other officials briefed on the investigation have told AP previously the possibility of a virus or other pathogen has not been high on their list of suspicions.

"We are not much further ahead than we were in finding out why this occurred," Undersecretary of State Steve Goldstein said after the hearing. Still, he insisted that President Raul Castro's government "knows what occurred" and refuses to tell the U.S.

A child holds a Cuban flag marking the 59th anniversary of the arrival of Fidel Castro and his rebel army in Havana, Cuba, Jan. 8. (Ramon Espinosa/Associated Press)

Cuba has repeatedly and adamantly denied involvement or knowledge of any attacks. Josefina Vidal, the country's top diplomat for U.S. affairs, described Tuesday's hearing as an irresponsible effort to advance an anti-Cuban political agenda, arguing that "months of investigation have shown that there has been no attack of any sort."

"The biggest victim of today's hearing was the truth," Vidal told reporters in Havana.

Several Cuban diplomats were ordered to leave the U.S. last October

But Brown said even if the strange sounds heard by the vast majority of the 24 "medically confirmed" U.S. patients didn't cause the damage, the sounds weren't necessarily unrelated.

"The acoustic element could be used as a masking piece," Brown told the senators. Last month, the AP reported investigators believed the sound might have been emitted by some type of device that harmed the Americans in another way.

Dr. Charles Rosenfarb, the State Department's chief doctor, also dismissed speculation that the illnesses were psychosomatic. He said there were "exact findings" on precise, objective medical tests that can't be easily faked.

"The findings suggest this is not an episode of mass hysteria," Rosenfarb said.

The AP has previously reported that brain abnormalities were found in the Americans who fell ill, specifically changes to the white matter tracts that form the brain's internal communication system.

It was unclear how many patients had those abnormalities. But Rosenfarb said of the 80 embassy workers and spouses tested between February and April of 2017, 16 had symptoms and "medically verifiable clinical findings" consistent with mild traumatic brain injury. The actual total is likely higher, because more Americans were tested later following additional suspected attacks.

Rosenfarb said some symptoms started "within minutes to hours of the event," including "sharp, localized ear pain," extreme fatigue, and problems focusing visually. He said for many patients, symptoms disappeared days or weeks later, but they later developed other, persistent issues including problems with memory and concentration. Hearing loss in just one ear and problems sleeping and balancing also lingered.

From the Associated Press: Following months of investigation and four FBI trips to Havana, an interim report from the bureau's Operational Technology Division says the probe has uncovered no evidence that sound waves could have damaged the Americans' health. —@JeffFlake

Of the 24 patients, 10 have returned to work at least part-time, Rosenfarb said. The rest are still in treatment.

"At this time we are unable to state whether or not the injuries may result in adverse long-term consequences to the individuals' future health or functional abilities," Rosenfarb said.

How does the U.S. know their injuries resulted from deliberate attacks? Officials said investigators, working with top academic experts, considered every possible benign explanation, such as environmental toxins. They said each had a major hole that couldn't be explained and was ultimately ruled out. In July 2017, investigators determined some "non-natural" cause must be to blame.

But what could it be? The officials told senators the U.S. government knew of no weapon, sonic or otherwise, that could produce the effects seen in the Cuba patients.

With so many unanswered questions, the Cuba mystery has become a new front in the decades-old political battle between proponents and opponents of closer ties between the U.S. and Cuba, two countries estranged for a half-century until relations were restored under President Barack Obama in 2015.

Sen. Jeff Flake, a supporter of rapprochement with Cuba, has claimed there's no evidence anyone was attacked and that the U.S. should reverse its decision to withdraw most of its diplomats from Havana. That drew an angry response from fellow Republican Sen. Marco Rubio, a vocal critic of Castro's government.

Sen. Cory Gardner of Colorado and Sen. Jeff Flake of Arizona, both Republicans, confer as the Senate foreign relations subcommittee on Tuesday examined incidents involving American diplomats in Havana. (J. Scott Applewhite/Associated Press)

"Imagine you are one of these people who are out there working on our behalf, who have now suffered from these injuries," Rubio said. "Not only is it demoralizing, I think it's incredibly unfair to them."

U.S. Secretary of State Rex Tillerson will convene an Accountability Review Board panel to evaluate cases in which U.S. diplomatic personnel or facilities have been damaged abroad, said Francisco Palmieri, acting assistant secretary of state for Western Hemisphere affairs.

An unspecified number of Canadians also were hit by mysterious symptoms in Havana, CBC News has reported, and have all recovered.

The Cubans have been said to be co-operative as Canada has sent an RCMP delegation and a Health Canada doctor to examine Canadian diplomats, who suffered symptoms such as nosebleeds to short-term memory loss.

As with their American counterparts, no evidence of wrongdoing has been announced.