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. ||||| According to medical records reviewed exclusively by CBS News, a U.S. doctor who evaluated American and Canadian diplomats working in Havana diagnosed them with conditions as serious as mild traumatic brain injury, and with likely damage to the central nervous system. 
 
 The diplomats complained about symptoms ranging from hearing loss and nausea to headaches and balance disorders after the State Department said "incidents" began affecting them beginning in late 2016. A source familiar with these incidents says officials are investigating whether the diplomats were targets of a type of sonic attack directed at their homes, which were provided by the Cuban government. The source says reports of more attacks affecting U.S. embassy workers on the island continue. 
 
 The doctor, one of several who reviewed their cases, included a warning in the medical records about the health risks of future exposures. The diplomats underwent comprehensive audiological evaluations and a battery of other tests. 
 
 In a statement received by CBS News from the University of Miami Health System, they confirmed its physicians were "consulted" by State on its diplomats in Cuba. 
 
 "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," read the statement from the university. 
 
 "The expertise of our physicians and researchers across a variety of fields naturally positions us to assist in these matters, and we consider it our obligation and responsibility to share that knowledge as needed. Regardless of the circumstances of providing medical care, maintaining patient privacy stands as a top priority for the University." 
 
 An American doctor also visited Havana in the spring to assess U.S. embassy workers, according to the source. 
 
 A number of diplomats have cut short their assignments in Cuba because of the attacks. 
 
 The source says American diplomats have also been subjected other types of harassment including vehicle vandalization, constant surveillance, and home break-ins. 
 
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 Although the State Department says it hasn't identified a definitive source of the attacks, it has reminded Cuban authorities of their international obligations to protect diplomats. The U.S. also expelled two Cuban embassy officials in Washington May 23. 
 
 "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 involved," Secretary of State Rex Tillerson told reporters earlier this month. 
 
 Cuba has denied any involvement in the attacks. 
 
 "We remain in regular contact with the Cuban government to emphasize that we take these incidents very seriously and to resolve this matter in a satisfactory manner," a State Department spokesperson for Western Hemisphere Affairs tells CBS News. 
 
 But as the U.S. pledges publicly to hold Cuba accountable for the protection of its diplomats, there are signs the U.S. may be privately moving forward with plans to make it easier for Americans to visit and do business with the country. 
 
 On Tuesday, the Treasury Department's Office of Foreign Assets Control removed hundreds of internet domain names primarily focused on travel to Cuba from its Specially Designated Nationals and Blocked Persons list. The people, groups and entities on this list are restricted from doing business in the U.S. 
 
 "While there may be a legal basis for the removal of the domain names from the SDN/BP list, to remove so many at one moment has optical significance," says John Kavulich, president of the New York-based U.S.-Cuba Trade and Economic Council. It "is demonstrative of the distance between political rhetoric and practical implementation of policy." 
 
 But a Treasury Department official explained to CBS News that OFAC's removal of the domain names may be misconstrued as more significant than it actually is. A single company was delisted Tuesday, and that company had 60 websites with "countless domain names," the official said. Delisting that one company meant including all of the websites and domains associated with the company.
– 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.)