Image copyright Reuters Image caption Gina Haspel, Director of the FBI, said the bureau was investigating the symptoms
The FBI has announced it will start offering counselling to employees and family members who were sickened by an illness thought to be the “Havana Syndrome”.
The announcement came after a woman in California and two US soldiers suffered from what became known as the “Cuban health crisis”.
However, Secretary of State Mike Pompeo said there was no evidence to support the finding.
The Associated Press news agency first reported on the virus on Wednesday.
The news agency reported that 83 FBI employees and at least three families of FBI employees were believed to have contracted the illness, according to two former US government officials.
The man in California, who is named as Robert Hauter, is among the victims, his attorney told the AP, the agency reported.
A US soldier in Afghanistan and three Marines in Hawaii were also infected with the illness, AP reported.
Humboldt General Hospital said that Robert Emmons, the 63-year-old man, had been a patient there between 1 July and 2 August before passing away.
The two soldier cases were described as the first known confirmed cases of “a highly infectious and complex illness,” according to US Army health officials.
The US Defence Department has said that “two or three” of its military personnel who were serving in Afghanistan also developed an unexplained illness.
Image copyright Getty Images Image caption Allegations of a US military “health crisis” in Cuba come just days after President Donald Trump launched a military strike against Syria
In a statement, Defence Department officials said: “These individuals experienced an unexplained health problem requiring hospitalisation.”
The other men were described as receiving medical care in civilian settings.
At least one of the soldiers, Cory Eastlake, a former Green Beret and special forces medic who served from 2000 to 2006, was kept out of duty for two months to have tests, including blood tests, in preparation for possible symptoms being linked to the illness.
No charges were filed against him because he suffered no criminal misconduct.
Health officials have not directly tied the “Havana Syndrome” to any specific case, and the scientific community remains divided on the cause.
The World Health Organization said that “two to three cases” of acute decompensated neuropathy – a specific type of neurological disease thought to be caused by a nerve agent – were diagnosed across Europe, Asia and Latin America between March and June.
The US military said two and a half cases of unexplained “acute decompensated neuropathy” were also reported in the Army and Nato.
Analysis by Martin Patience, BBC News, New York
This is a critical time for the Cuban government.
In recent days the US has toughened its rhetoric against President Raul Castro and raised the possibility of “all options on the table”.
A State Department spokesman said there was no evidence that a particular source of nerve agent had been identified, but added that “US scientists continue to study the toxin.”
The US government reported that “several hundred thousand people” in Cuba have been exposed to a similar nerve agent used in the Japanese attack on Pearl Harbour.
Most of the people in the US are former political prisoners, as many of them were shot by the military during the invasion of Cuba, as well as those caught drinking the bootleg rum which could not be produced in the country under US sanctions.
While in Cuba, anyone seen disorientated could be initially diagnosed with “coercive amnesia”, that typically means a well known generalised stroke for which there was little or no physical evidence, especially an obvious loss of memory, according to the AP.
However, some people who contracted nerve effects even decades after the occupation are said to have experienced “accidental cephalopods”, like floating amoebas, floating in the water or going into strange, anomalous states such as trying to put socks on in their underwear.
In recent years, scientists have believed that a nerve agent could be detected in the food and water supply of Cuba, which is heavily dependent on Cuba’s state dairy, producing 80% of the nation’s milk.
At present, there are no patients testing positive for the nerve toxin, said health officials.
The NBC news network also reported that the US military has stopped adding milk and dairy products to the ration programme, which was designed to be available to everybody on the island, as to minimise any complications.