Financial police raided the Italian football federation and Napoli club offices Wednesday to seize documents involving the buying and selling of players and relations with agents.
Police carried out the raids at federation headquarters in Rome and at Napoli's training center in Castelvolturno on the order of Naples prosecutors.
The federation said police confiscated contracts, agent relations and other documents involving Napoli players, without citing names of the players involved.
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.
Napoli lawyer Mattia Grassani said the club has respected contract rules since Aurelio De Laurentiis took over the team in 2005, adding that the club was "completely transparent" with the police demands, the ANSA news agency reported.
Napoli hasn't won Serie A since Diego Maradona led it in 1987 and 1990. The team fell into financial difficulty and was relegated to the third division in 2004 but made it back to Serie A in 2007.