HAVANA – After the U.S. embassy opened in Cuba in 2015 not everyone on the communist island was willing to welcome the group of U.S. diplomats who moved to Havana.

The extent of the aggressions 19 U.S. diplomats faced while former President Barack Obama and Cuban President Raul Castro moved to normalize relations between the two Cold War foes is still unclear.

The U.S. State Department asked The University of Miami Health System for help with at least six of their employees earlier this year. Investigators later considered the possibility of a covert sonic device used to spy on the diplomats or to threaten them with a "health attack."

"It took time to figure out what it was and this is still ongoing," the U.S. State Department's Heather Nauert said during a news conference early in August. She also said, "we don't know exactly where [covert sonic device] came from."

U.S. laws protect the privacy of the diplomats' medical records. While most of the details remain a mystery, the State Department confirmed there were diplomats who suffered hearing loss. Their complaints included nausea, headaches and feelings of disorientation.

CNN reported at least one needed a hearing aid. The New York Times reported one had a more serious illness that involved a blood disorder. And on Wednesday, CBS News reported some diplomats suffered traumatic brain injuries and damage to the central nervous system during the "health attacks."

The U.S. expelled two Cuban diplomats in May 23. The Cuban Foreign Ministry released a statement saying "Cuba has never allowed or will it allow the Cuban territory to be used for any action against accredited diplomatic officials or their families, without exception."

Cubans have good relations with Canadians, so there was confusion when the Canadian Global Affairs spokeswoman Brianne Maxwell said in August that at least one of their diplomats in Cuba was also treated for hearing loss.

"The government is actively working -- including with U.S. and Cuban authorities -- to ascertain the cause," Maxwell said.

FBI agents are assisting the State Department's bureau of diplomatic security with the investigation. U.S. and Cuban officials will meet next month.

ABC News' Victor Oquendo contributed to this report.