The United States is asking its citizens to not visit Cuba and ordering more than half of its diplomats to leave the island, senior officials said on Friday, according to AP.

The move comes nearly a year after diplomats began to describe unexplained health problems in Cuba. At least 21 of them and their families reported injuries, some as serious as a traumatic brain injury and permanent hearing loss. US investigators still do not know what or who is behind the attacks, the AP report said.

While the US State Department initially called them “incidents” and avoided deeming them attacks, unidentified officials on Friday said the US had determined that they were “specific attacks” on American personnel in Cuba.

The announcement was first reported by CBS News, and comes days after US Secretary of State Rex Tillerson (pictured above) met Cuba’s Foreign Minister Bruno Eduardo Rodríguez Parrilla in Washington. Tillerson made the decision to draw down the embassy overnight, AP reported officials as saying, after considering other options, including a full embassy shutdown.

The officials demanded anonymity as the decision has yet to be announced, though the US informed Cuba of its decision early on Friday. Cuba’s embassy has not commented either.

The move will not only affect Cuba’s tourism, which is a critical component of its economy, but also set back the delicate reconciliation between the two countries that endured a half-century estrangement till 2015, when former President Barack Obama and Cuban President Raul Castro restored diplomatic ties. The Donald Trump administration has, so far, not pointedly blamed Cuba for perpetrating the attacks.

The mysterious attacks

The symptoms in the attacks vary from person to person. In addition to hearing loss and concussions, some experienced nausea, headaches and ear-ringing, and some now suffer from problems with concentration and word recall.

Initially, officials suspected sonic attacks, but the Federal Bureau of Investigation and other agencies that searched homes and hotels in Cuba where the incidents occurred found no devices, the AP report said.

The attack may have been the work of a country like Russia or a rogue government unit, The New York Times reported. A Canadian diplomat was also affected, and this has deepened the mystery as relations between Canada and Cuba have been warm.