Revelations provided by Mafia informer Tommaso Buscetta have brought Italian authorities to a breakthrough point in their investigations of the underworld activities of the Sicilian criminal organization, according to one of Italy's top Mafia investigators.

"We are now at a decisive point in our battle against the Mafia," said Judge Ferdinando Imposimato, who since 1983 has been conducting a major investigation into the laundering of Mafia drug trafficking profits in Rome and other parts of Italy. A year ago Imposimato's younger brother, Franco, was murdered by Mafia henchmen in an unsuccessful attempt to intimidate the Rome magistrate.

The judge's comments came only a few days after Palermo magistrates sent into "internal exile" Vito Ciancimino, 60, a former Christian Democratic mayor, councilman and city commissioner who was named by Buscetta as the Mafia's key local political contact.

The decision to confine Ciancimino temporarily to the seaside Sicilian town of Patti must be reviewed by a local court by Oct. 30. But the move, as well as an accompanying investigation of Ciancimino's affairs, is seen by many observers here as a first step by Italian authorities against the so-called "third level" that is believed to link Mafia bosses to Italian businessmen and politicians.

On Thursday a Palermo magistrate ordered arrest warrants for Giovanni Bruno, a builder with ties to Gaetano Badalamente, the 50-year-old Palermo Mafia boss arrested last spring in Spain who is to be extradited to the United States soon. Warrants were also issued for the mayor and entire ruling council of the Palermo suburb of Isola delle Femmine.

On the same day, Judge Giovanni Falcone left Palermo for Milan where, judicial sources say, he is following up other leads to Mafia business connections in that northern industrial city.

Imposimato's investigation, along with another looking into white-collar crime in Milan and the breakthrough investigation in Palermo by Judge Falcone, whose investigation led to Buscetta's arrest, are all based on the thesis that politicians and businessmen have helped the Mafia to "infiltrate" the Italian economy. This was allegedly done through investments designed to launder the profits from a multimillion-dollar drug smuggling business, as well as from other criminal activities.

According to Imposimato, 48, the continuing confessions of Buscetta -- known to his Mafia associates as "Don Masino" -- are important because they provide a foundation for the investigations into the recycling process begun in recent years.

"It is the first time a top-level Mafia figure has broken the wall of omerta [silence] and revealed the details of the organization's structure and operating mechanisms," said Imposimato, who compared the potential impact of Buscetta's revelations to the first confessions of leading Red Brigades terrorists when that organization first started to decline.

Italian press reports have said Buscetta told investigators of a "supercommission" made up of leaders of the Mafia and the Camorra crime organization of Naples that made most key decisions. Buscetta reportedly also has said several murders of top politicians or police officers were decided "in the political sphere."

According to judicial sources in Rome and Palermo, however, so far Buscetta's revelations have been most useful in efforts to trace the route of Mafia drug profits in the Sicilian and Italian economies.

A thorough investigation of Ciancimino is a perfect starting point for this inquiry, the sources said. The thin, mustachioed Sicilian was Palermo's public works commissioner for years. The decision to send him into internal exile this week stemmed from Buscetta's charges that the Corleonesi clan, the so-called "winning mafia," was relying on Ciancimino, now retired from politics, to help them win bids for a proposed massive Palermo building project.

The sources said a similar measure might be taken soon for two Palermo brothers who are suspected of Mafia friendships and who for decades have managed to control Sicily's quasi-state tax collection agency.

Buscetta reportedly also has supplied the names of the two men allegedly managing the contacts between the Mafia and part of the Neapolitan Camorra known as the "nuova famiglia" or new family, the head of which is currently being questioned in prison.

Saturday the Rome newspaper, Il Messaggero, identified the two men as a Sicilian, Alfredo Bono and a Neapolitan, Nunzio Barabrossa. The paper, quoting judicial sources, also said that Michele Greco, Italy's most powerful Mafia boss, now in hiding, worked out the Mafia- Camorra pact in 1977. Later Greco is said to have sent to Rome his number-seven man to organize ties with Rome underworld bosses, some of whom were later implicated in the P2 and Banco Ambrosiano scandals.

The problem, however, is that so far it seems that Buscetta has been unable or unwilling to name political names. But according to a top Italian magistrate, who asked that his name not be used, Italian investigators will probably get to the "third level" even without Buscetta's help. "It will just take longer," he said.