The two-year-old U.S. diplomatic relationship with Cuba was roiled Wednesday by what U.S. officials say was a string of bizarre incidents that left a group of American diplomats in Havana with severe hearing loss attributed to a covert sonic device. In the fall of 2016, a series of U.S. diplomats began suffering unexplained losses of hearing. Some of the diplomats' symptoms were so severe that they were forced to cancel their tours early. U.S. officials concluded that the diplomats had been exposed to an advanced device that operated outside the range of audible sound and had been deployed either inside or outside their residences. It was not immediately clear if the device was a weapon used in a deliberate attack, or had some other purpose. State Department spokeswoman Heather Nauert said the U.S. retaliated by expelling two Cuban diplomats from their embassy in Washington on May 23. The Cuban Foreign Ministry said it had been informed of the incidents on Feb. 17 and had launched an exhaustive, high-priority, urgent investigation." U.S. officials told The Associated Press that about five diplomats, several with spouses, had been affected and that no children had been involved. Cuba employs a state security apparatus that keeps many people under surveillance and U.S. diplomats are among the most closely monitored people on the island. The use of sonic devices to intentionally harm diplomats would be unprecedented.