Two Cuban diplomats have since been expelled from the U.S, confirmed State Department officials on Wednesday, although specific details surrounding the incident remain vague.
"We don't have any definitive answers about the source or the cause of what we consider to be incidents", Nauert said.
The FBI is now conducting further investigations into the reported incidents, and the State Department is prepared to take further action against the Cuban government if any evidence points in their direction.
United States diplomats in Cuba said they suffered occasional harassment for years after the restoration of limited ties with the communist government in the 1970s, harassment reciprocated by USA agents against Cuban diplomats in Washington.
The Cuban Foreign Ministry said Wednesday it had started an "exhaustive" investigation into the freaky incidents which the United States said had caused physical symptoms in its diplomats serving here. After months of investigation, US 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.
The symptoms ultimately led to the officials leaving Cuba, but the State Department would not specify what the physical symptoms were or when the Cuban officials left the US.
The hearing loss has been blamed on a secret sonic weapon and is just one a series of freaky incidents that have upset USA officials working in Cuba.
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Towards the end of previous year, U.S. diplomats began to suffer mysterious hearing loss, according to investigators.
The U.S. Embassy in Havana was reopened in 2015 following then-President Barack Obama's normalizing of relations with Cuba. "We asked two officials who were accredited at the Embassy of Cuba in the United States to depart the United States".
She added: "We had to bring some Americans home or some Americans chose to come home as a result of that". Along with probing possible Cuban responsibility, investigators are also exploring the possibility that the attack was carried out by a third country, like Russian Federation, operating without the Cuban government's knowledge.
Cuban officials have denied directing actions against the diplomats and have launched their own investigation into the allegations.
The Ministry of Foreign Affairs reaffirms that Cuba complies rigorously and seriously with all its obligations arising from the 1961 Vienna Convention on Diplomatic Relations in connection with the protection and inviolability of diplomatic agents and the premises of diplomatic missions.
Ms Nauert said the government was taking it "very seriously, and there is an investigation now under way".
The US has also since expelled two Washington-based Cuban diplomats.
An exterior view of the U.S. Embassy is seen in Havana, Cuba, June 19, 2017. Some had to go to the United States for treatment and one is thought to have become permanently deaf.
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