"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.
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.
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.
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.
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.