Canada says it is returning the families of its diplomats from Cuba as the cause of a mysterious illness that has afflicted Canadian and US officials remains unknown.

“Arrangements will be made to support our diplomatic staff and their families returning to Canada in the coming weeks, as well as for those families who had expected to be posted to Cuba this summer,” the government said in a statement on Monday.

According to a senior official whose name was not mentioned in the reports, 10 Canadian envoys and family members suffered symptoms including dizziness, headaches, and a lack of ability to concentrate.

Twenty seven underwent medical tests.

“The cause [of their symptoms] remains unknown but could be human-made,” the government said. Air and water tests “did not indicate anything that could point to a cause.”

‘Sonic attack unlikely to have taken place’

The government also said regular embassy operations would continue and that it did not believe there were any risks to Canadian tourists in Cuba.

More intense health problems, including probable brain trauma, have previously afflicted US diplomats in Cuba.

Canadian and US authorities initially suspected a “sonic attack” or a “mass psychosomatic incident,” but the Canadian official said those were “now considered unlikely.”

In the immediate aftermath of the alleged incident, unnamed US officials claimed that the diplomats had been victims of “acoustic attacks,” from what they said was a device that operated outside the audible range of sound. However, FBI agents dispatched to Cuba found no evidence of such an attack.

An investigation led by the Royal Canadian Mounted Police in collaboration with US and Cuban authorities is still underway a year after the health concerns first surfaced.

After decades of hostility, the US and Cuba restored ties and the US Embassy reopened in 2015. But relations have strained over the ailments affecting the American diplomats.