Bruno Contrada, a crucial figure in Palermo in the 1970s and 1980s who went on to occupy a senior position in Italy's intelligence services despite persistent doubts about his Mafia connections, was found guilty of passing on information to help mobsters evade capture and plan bomb attacks against their enemies.
His conviction following a two-year trial is the biggest victory yet for prosecutors seeking to unravel the web of links between corrupt state officials and organised crime in Sicily.
In particular, they have established an important precedent for the admission of evidence from former Mafiosi who have chosen to collaborate with the authorities. The case against Mr Andreotti, who is accused of being Cosa Nostra's godfather in Rome, rests on the testimony of many of the same Mafia turncoats who condemned Mr Contrada.
Defence lawyers in both the Andreotti and the Contrada cases have tried to accuse the turncoats of mounting a political plot against their clients in revenge for the break-up of their criminal cartels. But the Palermo court appears to have accepted the prosecutors' argument that their witnesses had genuinely broken with their past, managing to produce tallying stories even though they had no obvious means of communicating among themselves.
The turncoats, known as pentiti, have been breaking the Mafia's strict code of silence since the mid-1980s, but only began talking about their links with the police, the secret services and politicians after the post- war Italian order collapsed in 1992, saying it would have been too dangerous to open their mouths beforehand.
The judicial system has been slow to process their revelations in the courts. Indeed Mr Andreotti's trial, which began last September under the weight of nearly 100,000 pages of damning evidence, has convened barely half a dozen times and is not expected to finish for another two years.
No witness has been heard since early January because the court has been waiting for a lawyer to recover from an eye infection. Mr Andreotti is now expected to be back in court in Palermo in the next week or two, while in the central Italian city of Perugia a separate trial is due to start in which he is accused of ordering the murder of an investigative journalist, Mino Pecorelli, in 1979.
Reaction to the Contrada verdict suggests that passions are likely to remain high, however. Supporters of the former prime minister, Silvio Berlusconi, themselves targets for sporadic accusations of Mafia collusion, have attacked the verdict, saying it was based on circumstantial evidence.
Indeed, the issue is likely to feature prominently in the general election campaign since Mr Berlusconi, who is standing for re-election despite being on trial for corruption in Milan, has already accused sections of the judiciary of trying to target certain public figures, including himself, for political reasons.
Mr Contrada is now expected to return to jail pending an appeal.Reuse content