A string of mysterious illnesses ailing American diplomats at a U.S. consulate in southern China has prompted the State Department to evacuate some employees and their families as concerns grow that a “sonic weapon” is being deployed. The New York Times reports the State Department evacuated at least two afflicted Americans from Guangzhou, China this week after they reported hearing strange noises. It’s unclear how many diplomats and their families have exhibited symptoms, but, according to the Times, American officials are worried the diplomats have been subjected to targeted attacks involving odd sounds, leading to symptoms similar to those “following concussion or minor traumatic brain injury.”

The new incidents come weeks after the first employee case was reported in Guangzhou, which is home to some 170 American diplomats and employees. The growing number of reported symptoms is cause for concern as the situation closely resembles the experiences of American diplomats in Cuba, where, beginning in 2016, at least 24 embassy staff in Havana suffered brain injuries believed to be the result of exposure to similar sonic sounds. In Guangzhou, the consulate employees usually live in apartment complexes and that’s where the employees reported hearing the strange sounds.

The problem in Guangzhou appears to be more widespread than first believed and U.S. officials expect more diplomatic families to be evacuated. The State Department has already provided increased medical care, though it has not yet officially identified the cause of the unexplained illnesses, whose symptoms include headaches, chronic sleeplessness, nausea, hearing loss, and other concussion-like symptoms. One employee reported “subtle and vague, but abnormal, sensations of sound and pressure,” while another told the Washington Post he heard a sound that resembled “marbles bouncing and hitting a floor then rolling on an incline with a static sound.”