Story highlights The employees affected were not at the same place at the same time

They suffered a variety of physical symptoms since late 2016 resembling concussion

The FBI is now looking into the matter. "It's very strange," one official said

(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 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."

The 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.

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.

Read More