Tourists ride vintage American convertibles as they pass by the U.S. embassy in Havana in February 2016. (Desmond Boylan/AP)

The U.S. government expelled two Cuban diplomats in May, after Americans working at the U.S. Embassy in Havana suffered unexplained physical ailments, the State Department said Thursday.

A small number of Americans began reporting symptoms at the end of 2016, and a few either were removed for medical treatment in the United States or asked to leave, State Department officials said Thursday.

Spokeswoman Heather Nauert linked “incidents which have caused a variety of physical symptoms” to the decision to expel two Cubans, but she did not directly blame the Cuban government for harming the Americans.

“We don’t have any definitive answers about the source or the cause,” of symptoms she characterized as not life-threatening.

The FBI is conducting an investigation of the illnesses or symptoms reported by the State Department employees over several months. Further U.S. action could follow if that investigation points to Cuban government targeting of U.S. employees, officials said.

The State Department said the Cuban government has assured Washington that it is also investigating.

Harassment of U.S. diplomats in Cuba is common, and predates former president Barack Obama’s 2014 diplomatic opening to Cuba and the advent of a full-fledged U.S. embassy there.

“The Cuban government has a responsibility and an obligation,” to protect U.S. diplomats, Nauert said. “That is part of the reason why this is such a major concern of ours, why we take this so seriously.”

The State Department separately said in a statement that it had “reminded the Cuban government of its obligations under the Vienna Convention to protect our diplomats.”

The State Department would not provide further information on the kinds of symptoms reported, and would not give a precise number of Americans who left Cuba as a result. The two expulsions are considered reciprocal, however, suggesting that the number of Americans who left their posts is close to two.

“Initially, when they’d started reporting what I will just call symptoms, it took time to figure out what it was, and this is still ongoing,” Nauert said. “We’re monitoring it. We provide medical care and concern to those who believe that they have been affected by it.”

It was not clear why the expulsions were not made public by either country in May. President Trump announced the following month that he would roll back parts of the Obama opening, calling it “a bad deal.”

“Effective immediately, I am canceling the last administration’s completely one-sided deal with Cuba,” Trump said June 16.

He spoke in Miami, before an audience packed with members of South Florida’s Cuban American community who had opposed Obama’s normalization of relations with the communist government of President Raúl Castro.

The expulsions and the illnesses were first reported by CBS Radio News.

The details of Trump’s new policy are not yet clear. Trump ordered the Treasury and Commerce departments to draw up regulations, but White House officials said that actual changes would be months away.

Trump’s moves were opposed by U.S. business leaders and by both Democrats and Republicans in Congress.

Cuba’s government called Trump’s moves a “setback” in relations, and warned that “any strategy aimed at changing Cuba’s political, economic and social systems, whether through pressure or coercion, or employing more subtle methods, will be doomed to failure.”

Trump did not announce any change to the status of the U.S. Embassy in Havana, which replaced a smaller office that did not carry the same diplomatic status. Cuba’s small mission in Washington was also upgraded to an embassy.