Italian Prime Minister Mario Monti said on Wednesday that the country must keep working to fully investigate a murder case of a anti-mafia magistrate, his family and his colleague 20 years ago.

Monti made the remarks in Sicily at a commemoration ceremony to mark the 20th anniversary of Giovanni Falcone's death and Falcone's colleague Paolo Borsellino in 1992, according to Ansa news agency.

Falcone, his wife and three bodyguards were blown up by mafia with a massive bomb planted on a motorway near Palermo that left a crater three meters deep and 13 metres wide. Less than two months after Falcone's murder, Borsellino was also killed by mafia in a car-bomb attack in Palermo along with five of his bodyguards.

Many mobsters involved in the attacks had been brought to justice, among them was former Cosa Nostra head Salvatore Riina, who ordered a number of bomb attacks in Rome and Florence.

But doubts linger over whether Italian politicians were involved in negotiations with Mafia at the time and whether rogue elements in the nation's secret services had played a role in the bloodshed.

"There are no reasons of State that can justify delays in establishing the facts and finding who was responsible," Monti said at a ceremony at a Palermo garden devoted to mafia victims.

"Details have emerged in recent years that have led to the sentences being looked at again along with the missing pieces, which must be sought out by getting right down to the bottom,"he said.