At least 16 Americans have been affected by "incidents" appearing to target U.S. staff and government officials in Cuba, according to U.S. officials. 
 
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 State Department spokeswoman Heather Nauert provided the new number -- the highest figure yet -- in a briefing Thursday, noting that that number could change in what's described as an "unprecedented" situation. It's unclear if it includes government employees or their families as well. 
 
 "We have not seen this type of activity take place before," Nauert said Wednesday. "Those incidents have caused a variety of physical symptoms. ... We are working and have been working to provide our staff and U.S. government employees with the best medical attention that we can provide to them." 
 
 Officials said the U.S. could not say for sure who or what may be behind the incidents, but that they had stopped in the spring. No device or piece of equipment has been discovered yet, according to Nauert, but medical professionals and staff on the ground report that the harmful activity has ceased. 
 
 "We have not heard of any incidents" since the spring, an official told ABC News, although "some people may be experiencing symptoms ... because the symptoms are experienced at different times, because the symptoms are different in various people." 
 
 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. 
 
 "The investigation is ongoing, and we will continue to try to find the source of these incidents and the perpetrator," Nauert added Thursday. 
 
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 Nauert said the "first reported activity" had taken place in late December 2016. 
 
 "It took some time for people to be able to determine that, 'Yes, there is a pattern taking place here. Yes, there is something going on,'" she said Wednesday. 
 
 Sources told ABC News that some U.S. officials were exposed to a sonic device in Havana that caused serious health problems and physical symptoms. One State Department official said that some of those employees affected were aware that the incident was happening while it was happening, but others were not. 
 
 Experts told ABC News that sound waves above and below the range of human hearing could potentially cause permanent damage. 
 
 According to ABC affiliate WPLG in Miami, sources said the variety of symptoms suffered by those affected included headaches, vision issues, balance and walking issues, and memory loss. 
 
 Nauert said Wednesday officials had not all experienced the same symptoms and while some had been asked by the department to leave Cuba because "their condition necessitated that," others had chosen to stay. 
 
 "We have had our U.S. government employees go to Miami. ... Some of them had been medically evacuated in order to receive medical treatment and testing," she said. "We have brought medical professionals to our staff in Cuba to be able to treat them, to be able to test them." 
 
 The University of Miami told ABC News Wednesday that it had been contacted by the State Department and that it was looking into the situation. 
 
 "Like any top-notch academic medical center in the nation, the University of Miami is often consulted regarding complex health care issues or emerging diseases. In the case of U.S. diplomats, our physicians were consulted by the State Department," the university said in a statement. 
 
 U.S. officials told ABC News that the FBI is among those investigating on the ground in Havana, with the State Department's Bureau of Diplomatic Security taking the lead. 
 
 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 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. 
 
 Earlier in August, Secretary of State Rex Tillerson said, "We hold the Cuban authorities responsible for finding out who is carrying out these health attacks on not just our diplomats, but as you've seen now there are other cases with other diplomats as well." 
 
 While Tillerson used the term "attacks," Nauert continues to refer to the issue as a series of "incidents," citing the ongoing investigation. 
 
 "What has happened there is of great concern to the U.S. government," Nauert said Wednesday. "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. 
 
 ABC News' Serena Marshall, Josh Margolin, Mike Levine, Victor Oquendo, Seni Tienabeso and Kirit Radia contributed to this story. ||||| In this photo taken Aug. 14, 2015, a U.S. flag flies at the U.S. embassy in Havana, Cuba. American diplomats who served in Cuba have been diagnosed with mild traumatic brain injury following mysterious,... (Associated Press) 
 
 In this photo taken Aug. 14, 2015, a U.S. flag flies at the U.S. embassy in Havana, Cuba. American diplomats who served in Cuba have been diagnosed with mild traumatic brain injury following mysterious,... (Associated Press) 
 
 WASHINGTON (AP) — Mysterious incidents affecting the health of American diplomats in Cuba continued as recently as August, the United States said Friday, despite earlier U.S. assessments that the attacks had long stopped. The U.S. increased its tally of government personnel affected to 19. 
 
 The new U.S. disclosures came the same day that the union representing American diplomats said mild traumatic brain injury was among the diagnoses given to diplomats victimized in the attacks. In the most detailed account of the symptoms to date, the American Foreign Service Association said permanent hearing loss was another diagnosis, and that additional symptoms had included brain swelling, severe headaches, loss of balance and "cognitive disruption." 
 
 At the State Department, spokeswoman Heather Nauert said the U.S. was continually revising its assessments of the scope of the attacks as new information was obtained. She said the investigation had not been completed. 
 
 "We can confirm another incident which occurred last month and is now part of the investigation," Nauert said. 
 
 U.S. officials had previously said that the attacks, initially believed to be caused by a potential covert sonic device, had started in fall 2016 and continued until spring 2017. Last week, Nauert had said at least 16 Americans associated with the U.S. Embassy in Havana had been affected, but that the "incidents" were no longer occurring. 
 
 The evolving U.S. assessment indicated investigators were still far off from any thorough understanding of what transpired in the attacks, described by the U.S. as unprecedented. As the bizarre saga has unfolded, the U.S. has encouraged its diplomats to report any strange physical sensations. So it's unclear whether some symptoms being attributed to the attacks might actually be unrelated. 
 
 Still, the fact there was an incident as recently as August suggested the attacks likely continued long after the U.S. government became aware of them and ostensibly raised the issue with the Cuban government, creating even more uncertainty about the timeline and who was responsible. 
 
 Notably, the U.S. has avoided accusing Cuba's government of being behind the attacks. The U.S. did expel two Cuban diplomats, but the State Department emphasized that was in protest of the Cubans' failure to protect the safety of American diplomats while on their soil, not an indication the U.S. felt that Havana masterminded it. 
 
 U.S. investigators have been searching to identify a device that could have harmed the health of the diplomats, believed to have been attacked in their homes in Havana, but officials have said no device had been found. 
 
 One of the diplomats affected had arrived over the summer of 2017 to work at the U.S. Embassy and was later diagnosed with concussion-like symptoms, said a U.S. official, who declined to specify the symptoms that led the diplomat to report the situation. 
 
 And in Canada, a government official said that the Canadian government had first learned in March 2017 that one of its citizens was affected. Ottawa had previously confirmed that at least one Canadian diplomat was involved, but had not revealed any timeline for when it occurred or came to light. 
 
 Both the U.S. and Canadian officials demanded anonymity because they weren't authorized to comment publicly. 
 
 It's unclear whether Canadians were intentionally targeted or whether there could have been collateral damage from an attack aimed at Americans, given that diplomats from various countries often live in the same areas of a foreign capital. U.S. officials have said the Americans were targeted in their homes in Havana, not in the Embassy. 
 
 Canadian officials have been actively working with U.S. and Cuban authorities to ascertain the cause. A Cuban attack deliberately targeting Canadians would be even more confounding, given that Canada — unlike the U.S. — has long had friendly ties to Cuba. 
 
 The American Foreign Service Association, in describing the damage to diplomats' health, said it had met with or spoken to 10 diplomats affected, but did not specify how many of the 10 had been diagnosed with hearing loss or with mild traumatic brain injury, commonly called a concussion. 
 
 Yet the confirmation that at least some diplomats suffered brain injury suggested the attacks caused more serious damage than the hearing-related complaints that were initially reported. 
 
 "We can't rule out new cases as medical professionals continue to evaluate members of the embassy community," Nauert said. She added that the embassy has a medical officer and has been consistently providing care to those who have reported incidents. 
 
 Asked for further details about what the U.S. had learned about the cause or culprit in the attacks, the State Department said it had no more information to share. 
 
 Traumatic brain injury, or TBI, typically results from a bump, jolt or other external force that disrupts normal brain functioning, according to the U.S. Centers for Disease Control and Prevention. Short- and long-term effects can include changes to memory and reasoning, sight and balance, language abilities and emotions. 
 
 Not all traumatic brain injuries are the same. Doctors evaluate patients using various clinical metrics such as the Glasgow Coma Scale, in which a numerical score is used to classify TBIs as mild, moderate or severe. 
 
 "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 union said in a statement. 
 
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 Associated Press writers Michael Weissenstein in Havana, Rob Gillies in Toronto and Lauran Neergaard in Washington contributed to this report. 
 
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 Reach Josh Lederman on Twitter at http://twitter.com/joshledermanAP
– American diplomats treated after a series of unexplained incidents in Havana had health problems including mild traumatic brain injury and central nervous system damage, according to medical records seen by CBS News. The diplomats, some of whom ended up having to leave Cuba, were treated by an American doctor after complaining of symptoms including hearing loss, headaches, and loss of balance. Sources say the diplomats, who lived in homes provided by the Cuban government, began experiencing symptoms in the fall of 2016 after their residences were targeted with what American officials believe was some kind of advanced sonic weapon. Officials say at least 10 diplomats and embassy employees were affected. State Department spokeswoman Heather Nauert said Wednesday that the "unprecedented" incidents "caused a variety of physical symptoms." She said some diplomats were treated in Miami and others were treated by American medical personnel who traveled to Cuba. "What has happened there is of great concern to the US government," she said. "Let me just reassure you that this is a matter that we take very seriously." Officials tell ABC News that the FBI and the State Department are investigating in Cuba. Cuban authorities are assisting the investigation, the officials say. (At least one Canadian diplomat was also targeted.)