(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.

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.

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