A series of mysterious “incidents” has left Americans working at the United States embassy in Havana with medical “symptoms,” forcing them to return home and prompting the US government to expel a pair of Cuban diplomats from their own country’s embassy in Washington, DC.

State Department spokesperson Heather Nauert said Wednesday that officials first learned about the “incidents” at the US embassy in Cuba in late 2016. The incidents left multiple employees with “a variety of physical symptoms,” she said, and as a result the State Department “had to bring some Americans home.”

Speaking at a State Department press briefing, Nauert did not provide details about the “incidents” or the symptoms the Americans suffered, saying only that they were medical in nature and that diagnosing them “took time.”

However, the Associated Press, citing unnamed sources, reported Wednesday that the symptoms included hearing loss, which in some cases is believed to be permanent. The AP also reported that investigators are trying to determine if the Cuban government used sonic devices that create nonaudible noise to try to deafen the Americans.

“We don’t have any definitive answers about the source or the cause of what we consider to be incidents,” Nauert said.

The US is taking the situation “extremely seriously,” she added, and in May asked two Cubans working at the country’s embassy in Washington, DC, to leave the US. They have since complied.

“The Cuban government has a responsibility and an obligation under the Geneva Convention to protect our diplomats,” Nauert said, “so that is part of the reason why this is such a major concern of ours.”

The Cuban government had denied any role in the incidents, the AP reported.

The diplomatic kerfuffle comes at a time of heightened uncertainty over the relationship between the US and Cuba. Last year, the outgoing Obama administration reopened the US embassy in Havana after more than half a century, the most significant in a series of steps toward restoring diplomatic relations between the countries, which collapsed during the Cold War.

President Donald Trump, however, has taken a different approach, announcing in June that he would roll back Obama-era policies that aimed to make travel to Cuba easier for Americans.

The State Department did not answer questions from BuzzFeed News Wednesday, and Nauert did not say during the briefing if shifting policies toward Cuba were a factor in the incidents.

The FBI, which the AP reports is investigating the incidents, also declined to comment to BuzzFeed News Wednesday.

According to the AP, US embassy personnel in Havana live in housing that is owned and operated by the Cuban government. Last fall, about five diplomatic staffers and several spouses began experiencing hearing loss that was serious enough to prompt the investigation.

Nauert confirmed during Wednesday’s briefing that the investigation is ongoing and authorities are still trying to determine what afflicted US Embassy staffers.

“It is a cause of great concern for us,” she said. “It’s caused a variety of physical symptoms in these Americans who work for the US government. We take those incidents very seriously and there is an investigation currently underway.”