(ANSA) - Palermo, March 12 - New claims made by Mafia 'boss of bosses' Toto' Riina in tapped prison conversations leaked on Wednesday could shed new light on the yet-unsolved 1992 slaying of top anti-Mafia prosecutor Paolo Borsellino.

According to wiretaps of a prison conversation last year between Riina and another inmate, boss Alberto Lorusso from the Puglia-based Sacra Corona Unita mafia, the huge carbomb that killed Borsellino and his security detail in July 1992 outside his mother's apartment was detonated by the intercom the magistrate rang to get into the building.

Prosecutors in Caltanissetta led by Sergio Lari, who have opened a new probe on the carbomb attack, are investigating this latest claim made by Riina which would add a key piece in the puzzle on Borsellino's death as it is still unclear who detonated the bomb 22 years ago.

The wiretaps currently being examined by investigators are reportedly difficult to decipher and it is hard to understand whether the carbomb's explosion was provoked by Borsellino himself when he rang his mother's doorbell or whether the detonator allegedly placed inside the intercom was operated by Mafia boss Giuseppe Graviano, who was hiding nearby.

No Mafia turncoat so far has been able to tell investigators who detonated the bomb.

The latest mobsters-turned-informant to testify on Borsellino's murder, Gaspare Spatuzza and Fabio Tranchina, have also failed to answer to this key question.

Spatuzza however has said he saw a man close to the carbomb the day before the attack, when the vehicle was filled with explosives in a garage near the location of the attack in Palermo, and that he was not a Mafia affiliate but could have been the technician who placed the detonator, Rome daily La Repubblica reported on Wednesday.

Nicknamed 'the Beast', Riina is in jail for crimes including the 1992 slayings of Borsellino and another leading anti-Mafia prosecutor Giovanni Falcone two months earlier. He has gloated in tapped prison conversations over past murders and possible future ones.

In the wiretaps leaked on Wednesday, he bragged that the idea to place the detonator inside the intercom was a "stroke of genius".

Prosecutors in January entered into trial evidence other wiretaps of prison conversations between Riina and Lorusso.

The conversations held in November last year were admitted as evidence for an ongoing Palermo trial into alleged secret negotiations between the Italian State and the Mafia two decades ago. The Falcone and Borsellino murders were among the crimes that allegedly induced the State to enter into secret talks with Cosa Nostra two decades ago in a bid to stop attacks after a long campaign of violence that included blowing up judges and prosecutors with carbombs.

In the November conversations, Riina gloats, among other things, over the July 1983 carbombing murder of prosecutor Rocco Chinnici, who created the Antimafia Pool, laying the groundwork for the Maxi Trial against the Sicilian Mafia in 1986, and who frequently spoke out publicly against the Mafia.

He also threatened Palermo Prosecutor Nino Di Matteo in a wiretapped November 16 chat with Lorusso which investigators fear was in fact an order to kill the magistrate.

Di Matteo's security arrangements were increased as a consequence.

"Let's organize this thing," Riina said in the conversation.

"Let's make it real big," he added, threatening to kill Di Matteo "like tuna fish".

Though Riina has been in jail since 1993, he is believed to still have a big say in running Cosa Nostra.