BEIJING (AP) — China says it has not yet found any explanation for a U.S. government employee's report of abnormal sensations of sound and pressure while working in the southern city of Guangzhou.

The incident recalls the experiences of American diplomats who fell ill in Cuba last year, often after hearing a strange sound.

Chinese foreign ministry spokesman Lu Kang said at a regular briefing Thursday that China has begun a "careful investigation" and has already given the U.S. preliminary findings.

He added that the probe has not found "any reason or clue leading to the situation described by the U.S."

The State Department said Wednesday that the U.S. government employee was sent to the U.S. for additional testing after showing signs of a mild traumatic brain injury, commonly known as a concussion.