Data Theft By Ex Employee

Recently, when we do the system Audit we found one of our ex-employee copied and uploaded all company documents from our google drive database. In this regard, we need guidance on how we can proceed forward to take action on this ex-employee. Whether, I can send an official letter to him to explain to us why he as copied all our documents, or we can file a case in the cyber-crime or we have any other options to proceed further.