The bank's honorary president, Enrico Cuccia, and three other officials were notified of the investigation by magistrates in Ravenna.
The secretive bank dominates Italian business life, advising leading companies from Fiat to the Fininvest media empire of Prime Minister Silvio Berlusconi.
Mediobanca said that its managing director, Vincenzo Maranghi, and two directors also received official warnings that they were under suspicion for allegedly issuing false statements about Ferruzzi's accounts.
Under Italian law, an official warning does not mean charges will be brought. It is a warning to suspects that they should hire a lawyer.