New 'sonic' attack reported in Cuba, 19 Americans now affected

— -- Nineteen Americans are suffering from a range of symptoms, including mild traumatic brain injury and hearing loss, related to mysterious "sonic harassment" attacks in Cuba -- with a new incident reported just last month.

Previously, U.S. officials said the incidents started in December 2016 and ended this past spring. But State Department spokesperson Heather Nauert revealed Friday that a new incident occurred in August and is now part of the ongoing investigation.

"We can’t rule out new cases as medical professionals continue to evaluate members of the embassy community," warned Nauert, who has described the situation as "unprecedented."

The U.S. government, including the FBI, continue to investigate who and what are behind the incidents, but with no firm answers so far.

The American Foreign Service Association said Friday that its representatives met this week in Washington, D.C., with Foreign Service Officers posted at the U.S. embassy in Havana who have faced diagnoses including mild traumatic brain injury and permanent hearing loss, but also loss of balance, severe headaches, cognitive disruption and brain swelling.

Traumatic brain injury is caused by a violent blow or jolt to the head or body that may cause temporary dysfunction of brain cells or more lasting damage, according to the Mayo Clinic. Symptoms can be immediate or appear days or weeks later, ranging from loss of consciousness or confusion to sensory problems, memory loss, or headache and nausea.

AFSA said they only met with 10 affected because the others were not available; the State Department has said that some of those affected have remained at their posts in Havana.

Sources have told ABC News that some U.S. officials were exposed to a sonic device in Havana that caused serious health problems and physical symptoms. Sound waves above and below the range of human hearing could potentially cause permanent damage, medical experts have told ABC News.

No device or piece of equipment has been discovered yet, according to Nauert. Some of the affected Americans are still experiencing symptoms "because the symptoms are experienced at different times, because the symptoms are different in various people," according to a State Department official.

The Cuban government, which denies any involvement, is said to be cooperating with the ongoing U.S. investigation, but the two governments are not working together on the matter.

In May 2017, the State Department asked two Cuban officials working at the embassy in the United States to depart the country. The State Department said that the move was not a form of retaliation or a sign that the U.S. believes Cuba is behind the attack but rather to punish Cuba for its failure to keep American diplomats safe -- something it is obligated to do under an international treaty known as the Vienna Convention.

AFSA is encouraging the State Department and 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."

"What has happened there is of great concern to the U.S. government," Nauert has said, defending the U.S.'s response. "Let me just reassure you that this is a matter that we take very seriously.... It is a huge priority for us and we're trying to get them all the care that they need."

There have been no reports of other embassies experiencing this, a senior State Department official said.