A mysterious illness that hit dozens of American and Canadian diplomats in 2016 in Havana, Cuba, is likely connected to insecticides sprayed to fight Zika virus, according to a new study from a team of scientists in Canada.

Government personnel complained about symptoms including vertigo, ringing in the ears, pain, blurred vision, dizziness and memory and concentration problems. Leaders at the US Department of State were concerned that personnel were victims of an “acoustic attack” by sonic devices. The mysterious illness came to be known as “Havana syndrome.”

The diplomats have been studied by federal agencies and government scientists. They’ve been at the center of a Senate hearing and investigations. The Cuban government vigorously denied any involvement in the incidents. President Donald Trump said in 2017 that he believed Cubans were “responsible” for the diplomats getting sick. In March 2018, the State Department announced it was keeping staff at its embassy in Cuba at the minimum level required to perform “core diplomatic and consular functions” due to concerns about “health attacks” on staff.

Testing on 26 Canadian diplomats in the period from August 2018 to February 2019 raised the possibility of “overexposure to cholinesterase inhibitors,” possibly through insecticides. Cholinesterase is an enzyme required for the proper functioning of the nervous systems.

“While the source of exposure to toxins of the cholinesterase inhibitor family is not yet confirmed in our study, the use of insecticides readily and evidentially suggests itself,” the study said. “Importantly, certain chemical classes of pesticides, such as organophosphates and carbamates, work against insects by inhibiting the action of cholinesterase, but can also be poisonous to humans.

The research was funded by Global Affairs Canada and has not yet been published, but is undergoing the peer review process. Researchers shared the study with CNN.

Tests included cognitive assessments, self-reported symptom questionnaires, blood tests, brain imaging and a medical history. When 24 of the original test subjects were retested 200 days later, doctors said there was “cognitive, vestibular, and oculomotor abnormalities that suggested a ‘sustained injury to widespread brain networks,’ ” according to the study.

Mass-spectrometry tests confirmed the presence of cholinesterase-inhibiting insecticides, including Temephos, an organophosphorus that is used in Cuba to fight mosquitoes. The tests also found 3-phenoxybenzoic acid, a common insecticidal metabolite.

Temephos was found in 6 of the 10 remotely exposed individuals, compared to one recently exposed individual and it didn’t show up in any of the patients that were unexposed. (People considered “remotely exposed” were those tested one to 19 months after returning. “Recently” exposed people were tested within one month of their return.) The 3-phenoxybenzoic acid was found in the majority of the exposed individuals who were tested.

The US Environmental Protection Agency canceled the use of Temephos in the United States in 2011, but the chemical is still used in Cuba and in some other countries.

The researchers wrote that, at the time, Cuba had well-documented efforts underway to stop the spread of the Zika virus, including mass indoor and outdoor fumigations. Embassy records also confirmed an increase in the number of times people sprayed for mosquitoes at the Canadian office and at staff homes in January 2017, which coincided with the time when people were reporting symptoms.

Zika virus disease is an illness spread through mosquito bites. It can cause birth defects and other neurological defects. After outbreaks began in 2015, the US Centers for Disease Control and Prevention activated its emergency response to Zika in January 2016, and deactivated it in September 2017.

A call to the US Department of State about the study was not returned.

One of the authors of the study said this is just the beginning of the investigation, but he hopes that what they learn will be able to help protect public health.

“It took us many months to get to this conclusion, and there is lots of research to be done still, but this is the beginning of an answer to what happened, we think,” said co-author Dr. Alon Friedman, a professor of neuroscience and the Dennis Chair in Epilepsy Research in the departments of medical neuroscience and pediatrics at Dalhousie University in Halifax, Nova Scotia.

Friedman said researchers hope to work with Cuban scientists to see if others in the area were impacted.

“There is a lot we don’t know about how much we can expose people to these chemicals and what are toxic levels, or if the damage in the brain is reversible,” said Friedman. “But it’s not called a neurotoxin for nothing. The hint is in the name.”