WASHINGTON —The two-year-old U.S. diplomatic relationship with Cuba was roiled Wednesday by what U.S. officials believe was a string of bizarre attacks on a group of American diplomats in Havana with a covert sonic weapon that left the victims with severe hearing loss.

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 re-establishment of diplomatic relations.

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 were attacked with an advanced sonic weapon that operated outside the range of audible sound and had been deployed either inside or outside their residences.

The U.S. retaliated by expelling two Cuban diplomats from Washington in May, State Department spokeswoman Heather Nauert said.

U.S. officials told The Associated Press that about five diplomats, several with spouses, had been affected and that no children had been involved. The FBI and Diplomatic Security Service are investigating.

Cuba employs a state security apparatus that keeps untold numbers of people under surveillance and U.S. diplomats are among the most closely monitored people on the island. Like virtually all foreign diplomats in Cuba, the victims of the attacks lived in housing owned and maintained by the Cuban government.

However, officials familiar with the probe said that investigators were looking into the possibilities that the attack was carried out by a third country such as Russia, possibly operating without the knowledge of Cuba’s formal chain of command.

The U.S. officials weren’t authorized to discuss the investigation publicly and spoke on condition of anonymity. Cuban officials declined to comment on the incident.

Nauert said that investigators did not yet have a definitive explanation for the incidents but stressed they take them “very seriously.”