The documents reportedly allege ties between the Naples mafia or Camorra and the former president of the province of Naples, Luigi Cesaro, who is now an MP for ex-premier Silvio Berlusconi's conservative Forza Italia party.
The paramilitary Carabinieri police officer, named as Giuseppe Iannini, was put under house arrest and faces charges of abuse of office.
His arrest came after an investigation launched in 2014 when police found secret anti-mafia probe documents on a pen drive at Cosentino's home in Caserta, north of Naples, including testimonies from informants.
Cosentino is appealing a four-year jail term he received in June for bribing a guard in Naples' Secondigliano prison to allow him unauthorised visits and other perks during his brief incarceration there in 2013.
Anti-mafia prosecutors put Cosentino and his brothers under investigation in 2014 for abetting the Camorra's powerful Casalesi's extortion racket.
A a member of parliament until 2013 for Forza Italia and its Campania regional coordinator until 2012, Cosentino served as junior economy minister from 2008-10 in the last Berlusconi government.
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