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, according to officials with knowledge of the investigation into the case. Several of the diplomats were recent arrivals at the embassy, which reopened in 2015 as part of former President Barack Obama's reestablishment of diplomatic relations with Cuba.

Some of the diplomats' symptoms were so severe that they were forced to cancel their tours early and return to the United States, officials said. After months of investigation, 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.

The U.S. officials weren't authorized to discuss the investigation publicly and spoke on condition of anonymity.

State Department spokeswoman Heather Nauert said the U.S. retaliated by expelling two Cuban diplomats from their embassy in Washington on May 23. She did not say how many U.S. diplomats were affected or confirm they had suffered hearing loss, saying only that they had "a variety of physical symptoms."