State Department spokeswoman Heather Nauert said in a statement late Friday that 19 Americans are now confirmed to have been affected, up from 16 reported last month. The Trump administration has not blamed the Cuban government for what the union representing Foreign Service officers called "sonic harassment attacks" dating to late 2016.
"We can confirm another incident which occurred last month and is now part of the investigation," Nauert said.
The State Department did not provide details of the event or say whether it occurred before or after the existence of the earlier incidents was reported in August.
"We can't rule out new cases as medical professionals continue to evaluate" diplomats and their families, Nauert said.
The American Foreign Service Association said it has met or spoken with 10 victims since the health problems came to light last month. The health concerns were revealed only when the State Department said in August that it had expelled two Cuban diplomats as a rebuke to the Cuban government.
The Trump administration says the expulsions were a protest of Cuba's failure to protect diplomats as required under the Vienna Conventions. The State Department has not explained why it did not make the expulsions public when they occurred in May.
"AFSA strongly encourages the Department of State and the U.S. Government to do everything possible to provide appropriate care for those affected, and to work to ensure that these incidents cease and are not repeated," the group said in a statement.
U.S. officials have said the Americans were harmed by an unknown sonic device or attack that damaged their hearing and caused other health problems. The injuries occurred while the Americans were serving at the U.S. Embassy in Havana and living in housing provided by the Cuban government.
"We're not assigning responsibility at this point. We don't know who the perpetrator was of these incidents," Nauert said last month.
The Cuban government has denied harming diplomats and is cooperating with an FBI investigation, officials said.
AFSA's statement provides the most complete public view yet of the range of symptoms suffered by the Americans, none of whom have spoken publicly.
"Diagnoses include mild traumatic brain injury and permanent hearing loss, with such additional symptoms as loss of balance, severe headaches, cognitive disruption, and brain swelling," AFSA said.
CBS had reported many of those diagnoses on the basis of medical records it obtained, but the State Department would not confirm the information. The State Department at first would say only that the Americans suffered non-life-threatening "symptoms." Secretary of State Rex Tillerson later confirmed that hearing damage was among the effects.
AFSA's statement is the first indication that, at least for some, the hearing loss is likely to be permanent.