Officials said Francesco Fonti, a former member of Calabria's 'Ndrangheta crime group, had disclosed the location of the ship to authorities.
Fonti said he used explosives to sink the vessel, along with two others.
Silvestro Greco, the head of Calabria's environment agency, said: "If the turncoat who admits sinking this ship also says that he personally has sunk another two ships, even without taking into account other investigations that suggest more than 20 ships were sunk, the government must find these other two ships."
Al Jazeera's Claudio Lavanga said investigators believe the mafia began sinking ships when the European Union introduced wider restrictions on toxic waste disposal.
Lavanga said the restrictions have made the process of waste disposal expensive and lengthy.
Sebastiano Venneri, vice president of the environmental group Legambiente, said former members of the 'Ndrangheta mafia have said that the crime syndicate had been paid to sink ships with radioactive material for the last 20 years.