(CNN) -- The US believes several State Department employees at the US embassy in Havana were subjected to an "acoustic attack" using sonic devices that left at least two with such serious health problems they needed to be brought back to the US for treatment, several senior State Department officials told CNN.
A sophisticated device that operated outside the range of audible sound was deployed either inside or outside the residences of US diplomats living in Havana, according to three US officials.
A US government official told CNN that the who, where and when point to "an attack" -- the US is investigating whether a third country was involved as "payback" for actions the US has taken elsewhere and to "drive a wedge between the US and Cuba."
One official said the employees could have suffered permanent hearing loss as a result.
The employees affected were not at the same place at the same time, but suffered a variety of physical symptoms since late 2016 which resembled concussions.
The State Department raised the incidents with the Cuban government over the course of several months and sent medical personnel to Havana, but have not been able to determine exactly what happened.
"It can be quite serious," one official told CNN. "We have worked with the Cubans to try and find out what is going on. They insist they don't know, but it has been very worrying and troublesome."
"It's very strange," one official said.
The FBI is now investigating with Cuba's cooperation and they will allow FBI agents onto the island according to a US government official.
It is not known to what degree Cuba was involved in the attack. A US official said elements within the Cuba government must have "facilitated" the attack on some level because Cuba's security services exercise such a tight grip over the country.
The source also said it is possible that the people behind the attack did not realize the extent of the damage they caused. One US diplomat now will need to use a hearing aid as a result of the injuries they suffered, according to the source.
The attacks have stopped "at least for now," according to the same source.
State Department spokeswoman Heather Nauert on Wednesday said that "some US government personnel" working at the US embassy in Havana, Cuba on official duty reported some incidents that were causing "physical symptoms." But she could not elaborate on the nature or cause of the incidents.
"Because there are a variety of symptoms, there could be a variety of sources," one US official said. "That is why we are being very careful here with what we say. There is a lot we still don't know."
For years US diplomats in Havana complained that they suffered harassment from Cuban officials and frequently had their homes and cars broken into. But diplomats said that after the US and Cuba restored full diplomatic ties in 2015, the campaign of harassment stopped.
Some of those affected chose to return to the US, said Nauert, prompting the administration to expel two Cuban diplomats from the embassy in Washington in May.
"The Cuban government has a responsibility and an obligation under the Geneva convention to protect our diplomats," Nauert told reporters, "so that is part of the reason why this is such a major concern of ours."
"We felt like we needed to respond to the Cubans and remind them of their responsibility under the Vienna convention," one of the officials said. The officials were not declared "persona non-grata" and may be allowed to return back to the United States if the matter is resolved.
Those affected were State Department employees, Nauert said, and no American civilians were affected. The State Department is taking these incidents "very seriously," she added, and is working to determine the cause and impact of the incidents.
A statement from the Cuban Ministry of Foreign Affairs on Wednesday categorically denied any Cuban involvement in the mistreatment of US diplomats in Cuba, and said the decision to expel Cuban diplomats was "unjustified and unfounded."
"The Ministry emphatically emphasizes that Cuba has never allowed ... Cuban territory to be used for any action against accredited diplomatic officials or their families, without exception," the statement said in Spanish.
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