The Associated Press reported Wednesday that persons familiar with the US probe, speaking anonymously, said one possibility being examined is whether potentially harmful sonic devices might have been secretly placed inside USA diplomats' residences in Havana, causing hearing loss.
State Department spokeswoman Heather Nauert has not elaborated on the nature of the reported incidents or the symptoms, except to say that they were not life threatening and that the matter is under investigation. "We don't have any definitive answers about the source or the cause of what we consider to be incidents".
While they were not life-threatening, she revealed a number of people had been brought home to the USA as a result.
The affected diplomats and their spouses began to experience symptoms of hearing loss so severe that an investigation was launched, and it was determined that they were at risk, the unnamed officials told AP. However, the person familiar with the United States probe said investigators were looking into whether the individuals were harmed outside the regular chain of command of the Cuban government.
It is not unprecedented or uncommon for American officials in Cuba to be harassed, the Post reported, pointing to the start of the practice emerging under the island's communist government in the 1970s. "The Department is taking these incidents very seriously and is working to determine the cause and impact of the incidents".
Cuban officials have pledged to investigate the incidents as well and take appropriate action, the officials told the Post.
There has been no immediate official reaction from Cuba.
The diplomatic row comes amid President Donald Trump rolling back the easing of restrictions against Cuba enabled by Barack Obama during the final two years of his presidency. The incidents resulted in two Cuban officials operating at the Cuban embassy in the United States being asked to leave the country on May 23.
"The safety and security of US citizens is our top priority", Nauert said, adding that "the Cuban government has a responsibility and obligation to protect our diplomats under the Geneva Convention".
In June, Trump announced that he was "canceling the last administration's completely one-sided deal with Cuba".