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.