Details of the alleged plot by Messina Denaro, one of the world's 10 most wanted men, on the prosecutor, Nino Di Matteo of Palermo, were revealed in letters to prosecutors a few days ago, the Italian news agency ANSA reported Tuesday.
Di Matteo is leading an investigation of police allegedly involved in the deaths of two Italian magistrates in 1992.
Denaro became the head of the Cosa Nostra in 2006 after the arrest of Bernardo Provenzano. Denaro has been a fugitive since 1993.
Italian police recently claimed to have severely damaged Denaro's power, instituting a "scorched earth" policy in which they seized millions of euros in assets and arrested a large number of his alleged associates.
Last year, Italy's top Mafia prosecutor, Pietro Grasso, claimed efforts against the criminal syndicate had been so successful "the Mafia effectively no longer has a No. 1."