Mystery shrouds 'incidents' in Cuba involving US embassy personnel Two Cuban diplomats were expelled from the U.S. as a result of the "incidents."

 -- The United States has expelled two Cuban diplomats following "incidents" that allegedly caused U.S. embassy personnel in Havana to suffer "physical symptoms," according to the U.S. Department of State.

A source familiar with the matter told ABC News on Thursday that the FBI is assisting in the ongoing federal investigation into the incidents. But what exactly happened remains a mystery, with State Department spokesperson Heather Nauert saying at press briefings this week there were still no "definitive answers about the source or the cause."

According to Nauert, some U.S. government personnel working in Cuba's capital city on behalf of the U.S. embassy there reported "what we consider to be incidents" that caused "a variety of physical symptoms." The State Department first learned of these incidents in late 2016, she said.

As a result, Nauert said, some of the affected staff had to leave Cuba for medical reasons while others came home on their own accord. She declined to specify the exact number of people.

The State Department "took further action" on May 23 and asked two Cuban officials working at the embassy in the United States to depart the country, according to Nauert.

"We take this situation very seriously," she told reporters Wednesday. "We had to bring some Americans home or some Americans chose to go home. And as a result of that, we’ve asked two Cubans to leave the United States and they have."

Nauert answered questions on the topic during a press briefing Wednesday without revealing many details. When asked specifically what the incidents were, Nauert replied, "That's what we're calling it. We don't know exactly what."

Nauert said the affected staff received medical examinations and are being monitored as officials continue to investigate the cause.

Any other personnel who believe they have also been affected are being monitored and provided with medical care.

"Initially, when they started reporting what I will just call symptoms, it took time to figure out what it was, and this is still ongoing," Nauert said. "The Cuban Government has a responsibility and an obligation under the Geneva Convention to protect our diplomats, so that is part of the reason why this is such a major concern of ours."

Nauert declined to provide further information other than to say the symptoms were "not life-threatening."

At a press briefing Thursday, Nauert reiterated that investigators "don't have the definitive answers yet" and they can't pin the blame on "any one individual or a country at this point."

However, sources told ABC News on Thursday that beginning last fall, some U.S. officials were exposed to a "sonic device" that caused hearing problems and other physical symptoms.

A senior official within the Trump administration told ABC News on Wednesday that Cuba was being "cooperative" with the investigation.

Cuba's Ministry of Foreign Affair issued a statement Wednesday night, saying it was informed of the alleged incidents by the U.S. Embassy in Havana and the U.S. Department of State on Feb. 17.

"Cuba took this issue with the utmost seriousness and acted expeditiously and professionally in order to clarify the facts of this situation, initiating a comprehensive, priority and urgent investigation on the indications of the highest level of the Cuban government," the ministry said in the statement. "To this end, it conveyed to the United States Embassy the need for information-sharing and proposed to establish cooperation between the competent authorities of both countries."

According to the Cuban Foreign Ministry, the U.S. Department of State informed the Cuban embassy in Washington, D.C., of its decision to expel two Cuban diplomats from the United States on May 23.

"This led to a protest by the Ministry of Foreign Affairs due to this unjustified and unsubstantiated decision," the ministry said in the statement. "The ministry categorically emphasizes that Cuba has never, nor would it ever, allow that the Cuban territory be used for any action against accredited diplomatic agents or their families, without exception."

In response to questions from ABC News on Thursday, the Canadian government indicated its diplomatic personnel had also been affected.

"We are aware of unusual symptoms affecting Canadian and U.S. diplomatic personnel and their families in Havana," Global Affairs Canada spokesperson Brianne Maxwell said in a statement. "The government is actively working -- including with U.S. and Cuban authorities – to ascertain the cause. At this time, we do not have any reason to believe Canadian tourists and other visitors could be affected."

Maxwell did not respond to questions about how many Canadian diplomatic personnel had been affected, what kinds of symptoms they suffered and whether they were hospitalized.

ABC News' Conor Finnegan, Benjamin Gittleson, Mike Levine and Josh Margolin contributed to this report.