Many more U.S. diplomatic personnel were injured in a series of mysterious acoustic attacks in Cuba than previously reported, the State Department confirmed Thursday.

State Department spokeswoman Heather Nauert told reporters that “at least 16” government employees stationed at the U.S. embassy in Havana were hurt in the incidents, which began in late 2016 and continued through the spring of this year. Previous reports indicated as many as 10 diplomatic staff had been affected.

Nauert said that all of the injured personnel have received treatment at medical facilities in the U.S. or Cuba. She did not indicate which of the victims are government officials and which are family members.

Reports surfaced earlier in August that the State Department had expelled two Cuban diplomats from Washington in response to “incidents” targeting U.S. personnel in Havana.

Department officials later said that embassy staffers had suffered a variety of physical symptoms from acoustic devices placed in or near their residences. Sources familiar with the matter told CNN that the devices emitted an inaudible frequency that caused caused nausea, headaches and hearing loss. In other instances, U.S. officials were subjected to deafeningly loud sounds similar to buzzing insects, sources said.

As a result of the attacks, two diplomats had to be evacuated to the U.S. for medical treatment for long-term hearing damage, and several others elected to leave their assignments early. Some of the victims have been diagnosed with conditions as serious as mild traumatic brain injury and possible damage to the central nervous system, CBS reported Wednesday.

The Canadian government has confirmed that some of its diplomatic staffers were also targeted in similar attacks. In June, five Canadian diplomats and family members reported symptoms matching those suffered by Americans, according to CNN. The timing suggests the attacks continued even as Cuban officials were investigating the earlier incidents.

Nauert said Thursday that Cuba is cooperating with the ongoing U.S. investigation. The Cuban government has denied responsibility for the attacks, saying in a statement that “Cuba has never, nor would it ever, allow that the Cuban territory be used for any action against accredited diplomatic agents or their families, without exception.”

The circumstances surrounding the acoustic attacks cast some doubt on Cuba’s motivation to harm American or Canadian diplomats.

The Obama administration re-established diplomatic ties with Cuba in 2015, ending nearly six decades of official hostility between Washington and Havana. While President Donald Trump has criticized the reopening as “one-sided” and placed some restrictions on travel and commerce with Cuba, he has maintained full diplomatic relations with Havana “in the hope that our countries can forge a much stronger and better path.”

Canada has enjoyed good diplomatic relations with Cuba for many years, forging deep economic ties to the island after even after the U.S. severed relations following Fidel Castro’s communist revolution in 1959.

U.S. investigators are looking into the possibility that a third country with an adversarial relationship with the U.S. — Russia, China, North Korea, Venezuela or Iran — is responsible for the attacks.

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