United States spies have been hit the hardest and most severely by mystery health attacks on diplomats in Havana, Cuba, the Associated Press discovered and reported on Monday. The attacks began shortly after Donald Trump was elected president in November. Officials in the U.S. realized something was amiss after intelligence operatives, working under diplomatic cover in Cuba, started reporting “bizarre sounds” and feeling strange physical symptoms, according to U.S. News & World Report

There is more that is uncertain about the source of the medical conditions that struck intelligence operatives than is known.

No one knows why or when the mystery attacks started, and only that U.S. spies, tied to the American Embassy in Havana, were the first to report medical symptoms, the Denver Post noted. The investigation is ongoing.

Mystery attacks of diplomatic community disrupting U.S.-Cuban relations

The 21 people suffering ill effects have been described by the Trump administration as U.S. Embassy personnel. According to the Denver Post, members of the diplomatic community are principally suffering the unexplained medical problems – related to a mystery source. The events in question lead U.S. officials to the conclusion that diplomats and their family members were specifically affected. The impact of the attacks is a disruption in U.S.-Cuban relations, the Post wrote.

Many of the initial reports of medical ailments derived from intelligence operatives assigned to the U.S. embassy in Cuba. Investigators immediately focused on ferreting explanations from within the world of counterespionage, the Associated Press (AP) reported.

U.S. spies most seriously affected by health problems tied to Embassy work

The medical problems vary among the 21 people afflicted, though U.S. spies reportedly have experienced the most acute effects. Two of the most serious conditions that hit people include hearing loss and brain injury.

The U.S Department of State has stated that all of the cases are “medically confirmed,” according to the AP.

A number of U.S. officials stated that it is not certain that all of the symptoms people are suffering can definitively be connected with attacks. Officials also said that with the heightened sense of alarm people working in the U.S. Embassy feel, there is also the possibility that some workers have mistakenly “attributed unrelated illnesses to attacks,” the AP noted.

FBI conducted sweeps for devices but found nothing causing health attacks

So far, the United States has not been able to identify a device related to the medical illnesses. The FBI has even conducted sweeps but the agency found nothing. In order to possibly discern patterns, investigators have come up with a map that details specific areas of Havana where the attacks reportedly happened.

There are three areas that are considered geographic clusters of the health attacks. The attacks primarily happened at the historic Hotel Capri and additional hotels and housing where U.S. diplomats live, the Denver Post reported.

U.S. Officials shied from stating health problems caused by ‘specific attacks’ targeting spies

When the attacks were first reported, U.S. officials shied from defining the situation as attacks. Rather, the government referred to the problem as incidents. Last Friday, however, the U.S. Department of State finally classified the problem as “specific attacks,” stating that the mysterious source is targeting American diplomats in Havana, the AP relayed.

Many of the U.S. spies have worked overseas for years and don’t have homes in the United States, which has created another problem since the United States is reducing personnel at the U.S.-Cuban embassy.

Diplomats are selling possessions, job hunting, and searching for housing in the U.S.

Cuba has categorically denied that it has any involvement or awareness of the cause of the attacks, according to the AP. There are U.S. government officials who think that the Cuban government is telling the truth. While Cuban officials have conveyed disappointment in the U.S. scaling back personnel in the country, Cuba has stated that it plans to keep cooperating as both countries investigate.