His name is Bernardo Provenzano and he was the undisputed head of the mafia, the successor to his friend Riina.

Neither man is permitted to talk to anyone. They are frisked several times a day and are under constant surveillance.

But the two men, both over 70, are still making the police nervous. Italian state prosecutors in Palermo, Sicily, say two postcards enclosed in envelopes arrived at the men's cells late last month. Each had the same message: la pace e finite - peacetime is over.

It appears that Provenzano's arrest has launched a battle for the Cosa Nostra throne. There are two known candidates for the position and each has been on Italy's most-wanted list for more than 20 years.

The first is Matteo Messina Denaro, who is reputed to be a merciless killer and wants to be known as a Latin lover. The second is Salvatore Lo Piccolo, boss of the Tommaso Natale mafia family from Palermo.

Lo Piccolo is the more determined of the two and is capable of using any means necessary to become the ultimate boss, prosecutors say. His plan is to eliminate the entire Corleone clan, which support Denaro as boss.

Lo Piccolo is not the absolute authority yet, but in Palermo his authority goes largely unchallenged. Under him the mafia has been more aggressive. Mafiosi are demanding that more store owners pay the pizzo - protection money, collected monthly and typically equivalent to 2 per cent of sales - and are not hesitating to use violence.

Lo Piccolo kills his rivals in broad daylight, prosecutors say. Those inside the prison, like Riina and Provenzano, are asking themselves difficult questions: who will support their families? Who will take care of them? In the past, the mafia used to donate some of its profits to the families of incarcerated bosses. Now that the criminal organisation no longer has a well-established hierarchy, these families don't see a cent of the mafia's money.

In this highly charged atmosphere, the words whispered by Provenzano to an investigator when he was arrested - 'You don't know what you've done' - are proving chillingly ominous. Provenzano knew that, in his absence, a power vacuum would form and the beaten-down clans would try to rise to power.

A bloody civil war seems imminent, prosecutors believe.

When Provenzano ruled, things were different. The mafia did not like publicity and violence was always the last option. Compromise, deals with businessmen and being friendly with politicians were the order of the day.

In a matter of only weeks, the situation in Sicily seems to have spun out of control.

Nicolo Ingarao, one of the most powerful bosses in Palermo and an ally of the Corleone clan, was shot dead two months ago in broad daylight by Lo Piccolo's men.

In the summer Lirio Abbate, a journalist with the national Ansa press agency, wrote a book about the men who helped hide Provenzano from the authorities. He received so many death threats that he was given a police escort.

In recent weeks, two members of the Catania mafia family were killed.

Giuseppe Di Lello, an Italian senator and former member of the anti-mafia squad, thinks Lo Piccolo will stop at nothing to gain power.

The war could even reach into the prisons. Two family bosses, Leoluca Bagarella and Benedetto 'Nitto' Santapaola, appeared to want to stop Lo Piccolo. Bagarella, a Corleone family boss, killed a police commissioner in a Palermo bar in 1995. Santapaola, a boss of the Catania clan, claimed to have killed more than 70 people.

Bagarella was arrested in 1995, Santapaola in 1993. When in prison, they switched cells - mafia prisoners are not allowed to finish their terms in the same cell. When they were transferred, police noticed that each man had left his wedding ring hanging on a nail in the wall.

This was no coincidence. Both men were very attached to their wedding rings and to their wives, both of whom met with untimely deaths. Bagarella's wife committed suicide; Santapaola's wife was killed in an ambush by a rival clan.

The authorities think the incident could signify a new accord that involves the entire prison network and possibly some outsiders - an agreement between the Corleone and Catania clans to co-operate with each other. In the light of the message on the two postcards, it is likely that the incarcerated members of the Corleone clan are willing to do anything to remedy the situation and contact colleagues in the outside world.

When the war will go to the next level is anyone's guess. But Sicilians are prepared for the worst.