US investigators are looking at a range of theories – including the possibility of a “viral” attack – to explain what may have sickened some American diplomats stationed in Havana, according to the state department.

US experts have yet to determine who or what was behind the mysterious illnesses that began occurring in late 2016 and heightened tensions between Cuba and the US. They have seen no evidence it was “an episode of mass hysteria” among the 24 affected US personnel and family members, a senior state department medical officer told a Senate hearing on Tuesday.

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 are conducting their own investigation and have dismissed as “science fiction” the notion that some kind of sonic weapon was used.

The administration of Donald Trump, which has partly rolled back a detente with Cuba, sharply reduced the US embassy staff in Havana in response to the incidents 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, US 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 probably would have caused an extremely loud noise in the area, which was not the case.

Johana Tablada, deputy director for North America at the Cuban ministry of foreign affairs, said on Twitter it was “regrettable to see officials of the State Department using the word attacks with ZERO evidence to prove it happened”.

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.

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.

Francisco Palmieri, acting assistant secretary of state for western hemisphere affairs, said Cuba was responsible for the security of US 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.”

The Cuban government has accused the Trump administration of slander for saying Havana was holding back information.

Senator Robert Menendez, a Cuban-American and Democrat, said Cuba had failed to meet its international obligations but also that the state department response had been “inadequate and troubling”.

The secretary of state, Rex Tillerson, will open a new high-level investigation into the matter, Palmieri said. It will be in addition to other US inquiries, including one by the FBI.

Canada has said several Canadians reported symptoms similar to the US 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 US embassy reopened in 2015 as part of moves to mend ties by Barack Obama and Raúl Castro. Relations have been strained since Trump took office.