(ANSA) - Milan, December 16 - Four Italian magistrates have been put under the "highest level" of police protection after threats concerning their investigations of the Cosa Nostra Mafia, Interior Minister Angelino Alfano told a parliamentary Anti-Mafia Commission on Monday. The increased protection was necessary after incidents of intimidation against the magistrates, said Alfano, who spoke only a month after former Cosa Nostra Mafia "boss of bosses" Toto' Riina issued further threats against Palermo deputy prosecutor Nino Di Matteo from prison. Alfano also told the parliamentary commission that criminal organizations remain "among the most serious factors (responsible) for under-development in (Italy's) South and the greatest threat to economic freedom," in productive areas of the country. The mafia in Italy has gained a stronghold in financial activities and continues to control parts of the country, giving the gangs "power and prestige," said Alfano. Reports in mid-November caused a sensation by quoting a prison guard as saying that Riina shouted for the death of justice officials, including Di Matteo, from his prison cell while watching reports of hearings into alleged negotiations between mobsters and the Italian government. Later intercepts of a conversation with another mob figure in prison with Riina show the Corleone Mafia boss uttered further threats against the deputy prosecutor. "Di Matteo must die...they're making me crazy," Riina had shouted, according to a guard quoted by La Repubblica newspaper. "Those guys have to die, if it's the last thing I do," added Riina.