U.S. officials said Wednesday they believe a string of bizarre attacks on a group of American diplomats in Havana was carried out 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 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 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 their embassy in Washington on May 23, 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.

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U.S. officials say their diplomats in Havana were attacked with an advanced sonic weapon. (Ramon Espinosa/Associated Press)

Cuba employs a massive state security apparatus that keeps hundreds and possibly thousands of people under constant surveillance. U.S. diplomats are among the most closely monitored people on the island.

Like 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 the Cuban chain of command that would normally have to approve such a highly sensitive operation.

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

Nauert said that investigators “don’t have any definitive answers about the source or the cause of what we consider to be incidents” but stressed they take them “very seriously,” as shown by the Cuban diplomats’ expulsions.

“We requested their departure as a reciprocal measure since some U.S. personnel’s assignments in Havana had to be curtailed due to these incidents,” she said. “Under the Vienna Convention, Cuba has an obligation to take measures to protect diplomats.”

Nauert said the department had reminded Cuba of its international obligation to protect foreign diplomats.

Harassment of U.S. diplomats in Cuba is not uncommon and dates to the restoration of limited ties with the communist government in the 1970s. But, if it happened, the use of sonic devices to intentionally harm diplomats would mark a new phase in harassment.

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Cuban President Raul Castro, right, lifts up the arm of President Barack Obama at the conclusion of their joint news conference at the Palace of the Revolution, in Havana, Cuba on March 21, 2016. (Ramon Espinosa/Associated Press)