United States President Donald Trump on Monday said he believed Cuba was responsible for the unexplained “attacks” on American government workers in the country, AP reported.

Trump contradicted his own State Department, which has repeatedly said it does not know what is causing their government workers’ illnesses. It wasn’t clear if Trump meant Cuba was behind the attacks, or had simply failed to keep Americans safe on its soil.

In September, the US ordered more than half of its diplomats to leave Cuba, a year after they began to describe unexplained health problems. At least 21 of them and their families reported traumatic brain injuries and permanent hearing loss. While US investigators are yet to find out what or who is behind this, it has so far avoided blaming President Raul Castro’s government.

“I do believe Cuba is responsible. I do believe that,” Trump told reporters in the Rose Garden on Monday, according to The New York Times. “It’s a very unusual attack, but I do believe Cuba is responsible, yes.” The US government had not used the word “attack” in this regard.

Soon after, in a cable sent to all overseas US diplomatic posts, the State Department said it has “not assigned blame to the Government of Cuba”, AP reported. “We are still investigating these attacks and do not know who or what is behind them.” Trump’s comments could increase tensions between the countries, which had only re-opened diplomatic relations in 2015 after half a century of estrangement.

The mysterious illnesses

The symptoms of illness have varied from person to person. In addition to hearing loss and concussions, some experienced nausea, headaches and ear-ringing, and some now suffer from problems with concentration and word recall.

Initially, officials suspected sonic attacks, but the Federal Bureau of Investigation and other agencies that searched homes and hotels in Cuba where the incidents occurred found no devices. A Canadian diplomat was also affected, and this has deepened the mystery as relations between Canada and Cuba have been warm.