The United States may again close its embassy in Cuba, which reopened two years ago after a half-century stand-off, following a series of mystery "health attacks" on its diplomats, Secretary of State Rex Tillerson said today. At least 21 members of the US mission in Havana and a smaller number of Canadians have suffered brain injuries and hearing loss in what have been reported as "acoustic attacks", although US officials say their origin remains unclear. The incidents began last year, and the latest was recorded in August, despite US authorities having complained to Cuban officials in February and having expelled two Cuban diplomats from Washington in May. Some of those hurt were evacuated to Florida and some treated in place. The mission remains open, and US officials have warned that Cuba is responsible for the safety of diplomats on its soil, without accusing them directly. With the injury toll continuing to rise, and no explanation for what Washington has called an "unprecedented incident," some US lawmakers have called for the embassy to be closed down once again. Asked about this on CBS News' Face the Nation, Tillerson did not rule this out. "We have it under evaluation. It's a very serious issue with respect to the harm that certain individuals have suffered," he said. "We've brought some of those people home," he added. "It's under review." US officials have told reporters they believe some kind of sonic device was used to covertly undermine the health of staff members at the mission, who began reporting sick last year. The American Foreign Service Association - the labor union representing US diplomats - spoke to 10 of those who received treatment and said their diagnoses included mild traumatic brain injury and permanent hearing loss. At least five Canadian diplomats and their families were also affected by "sonic attacks," though none suffered permanent injury, public broadcaster CBC reported Friday. Canada has said Cuban officials are not suspected. The Cuban foreign ministry has said it is cooperating with the US investigation into the "alleged incidents." On Thursday, State Department spokeswoman Heather Nauert confirmed the number of Americans hurt had risen to 21. "We hope that that number will not increase. We certainly can't count that out. We are having our people medically tested," she told reporters. "Our folks are able to leave Havana, leave Cuba, and return back home if they wish to do so - I think we call it compassionate curtailment or something like that - where they're able to switch out a job," she said. "The investigation into all of this is still underway. It is an aggressive investigation. and we will continue doing this until we find out who or what is responsible for this." The State Department, citing respect for the privacy of its employees, has not discussed the diplomats' symptoms publicly, but Nauert said the form of injury "can be different in different people." Relations between the United States and Cuba were restored by then president Barack Obama and his counterpart Raul Castro in 2015 after a half-century. But tensions mounted again after Obama's successor Donald Trump, who won many Cuban American votes by promising a tough line, rolled back detente.