Financial police have raided the Italian football federation and Napoli club offices to seize documents involving the buying and selling of players and relations with agents.
Italian media report the police carried out the raids at federation headquarters in Rome and at Napoli's training center in Castelvolturno on the order of Naples prosecutors.
Reports say police confiscated balance sheets, player contracts, agent relations and other documents.
The entire scope of the inquiry was not immediately clear.
Through six rounds, Napoli is tied for the Serie A lead with Juventus. The southern club visits PSV Eindhoven in the Europa League on Thursday.