Washington – More than a year since the first of 24 US diplomats and family members fell victim to a mystery attack in Havana, the United States is still at a loss to explain what happened.Only one thing is clear. The US government holds Cuba responsible, arguing that Raul Castro's authoritarian state must have either carried out the assaults or at least know who did.The issue has poisoned any attempt to move forward with the fragile detente achieved between the former Cold War foes, who re-opened their embassies and exchanged ambassadors in 2015 for the first time since 1961.But, as Washington continues to press Havana for action, there is one extremely important plank in their case missing: US officials simply do not know who or what caused their diplomats to fall ill with injuries resembling brain trauma.On Tuesday, in a hearing for the Senate Foreign Relations Committee, State Department officials said they would open a review board into their own handling of the issue, but could offered frustrated lawmakers little in the way of answers.Acoustic weapon?Initially officials suspected the Americans had been targeted by some sort of acoustic weapon, although in public senior officials were more cautious, speaking of "health attacks".Now, US press reports suggest that FBI agents dispatched to Havana have been able to find no evidence to support the acoustic or sonic weapon theory.Some in Washington have begun to wonder if now is the time to start sending non-essential staff back to the Havana embassy, since there's no proof of what happened.Secretary of State Rex Tillerson, however, rejects that idea, as long as the those behind the "health attacks" have not been identified and dealt with."We're not much ahead of where we were in finding out what occurred and we need to find that out," assistant secretary of state for public affairs Steve Goldstein told reporters."We believe that the Cuban government knows what occurred, so what we'd like them to do is to tell us what occurred, so that this doesn't happen again."We're not considering restoring staff," he added.The chairperson of the Senate committee that summoned the officials, Senator Marco Rubio, did not hide his frustration."Maybe it wasn't acoustic, maybe they used microwaves, but that is irrelevant," he told reporters on Capitol Hill."At the end of the day we know that there was an attack, that 24 Americans were hurt when they were working and living in Havana as part of the embassy."Earlier, addressing the committee and the official witnesses, Rubio had dismissed the idea that anyone other than a government could have been behind the incident."Whatever happened to these people happened as a result of some sophisticated technology that quite frankly is so sophisticated that we don't understand it," he said.The State Department officials agreed that Cuba, with its powerful surveillance state, must have either been behind the attacks itself or at least know who was.Microwave transmittersHavana has fiercely denied this, accusing the Americans of withholding information about the victims of the incidents that would allow it to fully investigate.The first suspected attack was traced back to November 2016, and the last two took place in mid-August 2017, after Washington had protested to Havana and withdrawn some staff.Charles Rosenfarb, a doctor and director of the State Department bureau of medical services, told senators the symptoms were mixed but consistent with brain trauma.The victims – at least 24 Americans, a mix of US embassy personnel and their dependents – suffered headaches, hearing loss, disorientation and some loss of cognitive ability.Some recovered from the most acute symptoms, Rosenfarb said, but the severity, range and recovery time was mixed and it's not yet clear whether any have suffered permanent injury."They associated the onset of these symptoms with unusual sounds or auditory sensations," Rosenfarb said."Various descriptions were given; a high-pitched sound, an incapacitating sound, a buffeting sensation akin to driving with one window open or just an intense pressure in one ear."Cold War MoscowMedical experts and FBI agents launched investigations and high-frequency recording devices were installed in diplomatic residences, allowing a mystery sound sample to be captured.But US scientists have no idea what kind of weapon or agent could cause the symptoms, or whether the sounds heard were a by-product of the attack or its means of delivery.Rosenfarb said the only similar case he was aware of involved suspicions during the Cold War before 1976 that US diplomats in Moscow were hurt by Russian microwave transmitters.But acting assistant sectary of state Francisco Palmieri refused to say in an unclassified hearing whether Washington has approached the Russians over the Cuba incident."There is a long history and pattern of Cuban harassment of diplomats in Havana," Palmieri said, declining to speculate on why or if Raul Castro's regime had chosen to escalate."In whatever case they are responsible for the safety and security of US diplomats stationed in Havana ... and they have failed to live up to that responsibility," he said.