South Korean prosecutors raided the country’s biggest cryptocurrency exchange, Upbit, on Thursday and Friday on suspicion of fraud, Yonhap news agency reported.

The agency did not give details. Neither Upbit nor the financial investigation team of the Seoul Southern District public prosecutors’ office was immediately available for comment.

Upbit sent a note to its clients that it is co-operating with the prosecutors and that it can guarantee that all its clients’ assets are secure.

Four executives from two other South Korean cryptocurrency exchanges were detained in April in connection with the alleged embezzlement of billions of won.