The inquiry centres on AC Prato, which plays in the Lega Pro league, amid suspicions managers "favoured the illegal entry into Italy of African minors, some of whom were then sold to superior clubs," the Gazzetta dello Sport said.
The daily said the offices of two unnamed Serie A teams and one Serie B team had been searched as part of the probe, but added that the teams were believed to have bought the players without knowing about the fraud.
Managers in Prato are also suspected of match-fixing, the reports said. Four people including two of the mangers were placed in preventative detention by police on Thursday.
The Lega Pro is the third professional division, below Serie A and B.
Police suspect those held of having helped young Africans, mainly Ivorians, to obtain false papers or fraudulent family reconciliation visas.