WASHINGTON/HAVANA (Reuters) - U.S. investigators are looking at a range of theories, including the possibility of a “viral” attack, to explain what may have sickened some American diplomats who were stationed in Havana, the State Department said on Tuesday.

FILE PHOTO: A man works outside of the U.S. Embassy in Havana, Cuba, September 29, 2017. REUTERS/Alexandre Meneghini

U.S. experts have yet to determine who or what was behind the mysterious illnesses that began occurring in late 2016 and heightened tensions between the old Cold War foes. They have seen no evidence it was “an episode of mass hysteria” among the 24 affected U.S. personnel and family members, a senior State Department medical officer told a Senate hearing.

State Department officials testified that it was “incomprehensible” Cuba’s Communist government would not have been aware of what happened or who was responsible, though they stopped short of assigning direct blame to Havana.

Cuban officials, who are conducting their own investigation, have denied any involvement or any knowledge of what was behind it. Cuba’s Foreign Ministry chief for U.S. Affairs Josefina Vidal sharply criticized the United States late on Tuesday for talking of “attacks” when they had no evidence of that.

“The first victim of today’s hearing was the truth,” she told a news conference in Havana, adding that Cuba rejected the U.S. politicization of the matter.

The administration of President Donald Trump, which has partly rolled back a detente with Cuba, responded last year by sharply drawing down U.S. embassy staff in Havana and in October expelled 15 Cuban diplomats.

Todd Brown, diplomatic security assistant director at the State Department, said that as well as the possibility of an acoustic or sonic attack, U.S investigators were considering whether people might have been deliberately exposed to a virus. But he offered no details or evidence.

“I do know other type of attacks are being considered in a connection with this,” he said. “There’s viral, there is ultrasound, there’s a range of things that the technical experts are looking at.”

Some experts have argued that an acoustic attack seems implausible, given that it likely would have caused an extremely loud noise in the area, which was not the case.

Cuban officials have dismissed as “science fiction” the notion that some kind of sonic weapon was used.

Lawmakers also asked whether rogue elements of the Cuban government or security services or a third party such as Russia might have been involved. The State Department officials said they could not address or speculate on such matters in a public hearing.

‘TRAUMATIC BRAIN INJURY’

Symptoms suffered by the diplomats have included hearing loss, tinnitus, vertigo, headaches and fatigue, a pattern consistent with “mild traumatic brain injury,” said Charles Rosenfarb, director of the State Department’s Bureau of Medical Services.

A U.S. official told Reuters the government will not send staff back to the U.S. Embassy in Havana yet. The United States pulled out more than half of its personnel there in September.

“I don’t think we can say categorically that we can guarantee that they would be safe from this (if staffers return),” Brown told the hearing, chaired by Republican Senator Marco Rubio, a Cuban-American and strong critic of Cuba.

Francisco Palmieri, acting assistant secretary of state for Western Hemisphere Affairs, said Cuba was responsible for the security of U.S. diplomats on the island “and they have failed to live up to that responsibility.”

Asked whether it was possible that the Cuban government would have been unaware of any attacks, he said: “I find it very difficult to believe that. Cuba is a security state, the Cuban government in general has a very tight lid on anything and everything that happens in that country.”

Senator Robert Menendez, a Cuban-American and ranking Democrat on the Western Hemisphere subcommittee, said Cuba had failed to meet its international obligations but also that the State Department response had been “inadequate and troubling.”

Secretary of State Rex Tillerson will open a new high-level investigation into the matter, convening an accountability review board, Palmieri said. It will be in addition to other U.S. probes, including one by the Federal Bureau of Investigation.

Canada has said several Canadians reported symptoms similar to the U.S. diplomats but it has not publicly ordered any evacuation of embassy staff in Havana.

After decades of hostility between the United States and Cuba, the U.S. Embassy reopened in 2015 as part of moves by former President Barack Obama and his Cuban counterpart, Raul Castro, to mend ties. Relations have been strained since Trump took office, saying Obama made too many concessions and reversing parts of the rapprochement.