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.