Naples, January 29 - Marcello Dell'Utri, a Mafia-linked former close aide of ex-premier Silvio Berlusconi, was placed under investigation Tuesday in a probe into the alleged theft of antique books from the important Girolamini library in Naples. Dell'Utri, a Senator with Berlusconi's People of Freedom (Pdl) party on trial for allegedly helping Cosa Nostra and a noted bibliophile, is believed to have taken possession of several of the 1,500 books allegedly stolen from the library by its former head, Marino Massimo De Caro. Two people, a book-binder in Bologna who allegedly cancelled the Girolamini imprint from the books and a 'runner' who allegedly linked the thieves with buyers, were arrested while arrest warrants were served on four people already in custody. The four were detained last April when police blocked 500 volumes set for auction in Munich, Germany. All six now face charges of conspiracy, forgery and receipt of stolen goods. Investigations began in April 2012 after media reported that around 1,500 precious books were missing from the library, then under the direction of De Caro, a former advisor to the culture ministry. The probe has reportedly revealed the existence of an association dedicated to the systematic depletion of the contents of the library founded in 1586 and known particularly for its vast collection of writings on philosophy and theology. Meanwhile searches continued across Italy to locate numerous volumes that are still missing and which may have ended up on the antique book market. Earlier this month Dell'Utri was removed from the PdL's list of candidates in upcoming general elections due to concern within the party over the impact among voters of the criminal proceedings against him. He is currently facing justice in Palermo for allegedly helping the Mafia after Italy's Supreme Court of Cassation overturned an earlier nine-year conviction and ordered a retrial on grounds that his legal rights had not been observed. The chief prosecutor has asked for him to be sentenced to seven years in jail. Dell'Utri, whose cultural interests and vast book collection have often been praised by Berlusconi, is the former head of the media magnate's advertising arm who is credited with creating the three-time premier's first party, Forza Italia, in 1993, a year before it swept to victory in general elections.