Two employees at the US embassy in Havana have been sent home after suffering hearing loss from a silent "acoustic attack."
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.
The Cuban government says it had nothing to do with the attack.
Cuba employs a state security apparatus that keeps many people under surveillance and U.S. diplomats are among the most closely monitored people on the island. Like virtually all foreign diplomats in Cuba, the victims of the incidents lived in housing owned and maintained by the Cuban government.
However, officials familiar with the probe said investigators were looking into the possibilities that the incidents were carried out by a third country such as Russia, possibly operating without the knowledge of Cuba's formal chain of command.