(By Denis Greenan). Rome, June 27 - Italian President Giorgio Napolitano on Thursday called on investigators inside and outside Italy to get to the bottom of a deadly plane crash whose causes have been disputed for more than 30 years. Investigators "around the world", he said, must keep digging for the truth, hidden by years of cover-ups, behind the 1980 disaster off the southern island of Ustica that claimed 81 lives. "The memory of that tragic night and the innocent victims of the disaster underline the duty of all institutions to support ongoing investigations to ascertain responsibility - national and international - that remained covered by disturbing shadows and opacity," Napolitano said on the 33rd anniversary of the unexplained crash. The president also gave his condolences to the victims' families, led by association chief Daria Bonfetti, and praised the "constant dedication" with which they work to uncover the truth about the crash, which some have suggested was caused by a rogue missile. The association of relatives joined officials in urging the government to ratify a convention on judicial cooperation in criminal probes that they said would "give a major boost to efforts to get to the truth". Bologna Mayor Virginio Merola called for an "immediate" meeting with "senior cabinet officials". Ratifying the convention "could breathe new life into the investigations on what happened at Ustica," said Merola. Over the years, Italy has sought information from the United States, France, Belgium and Germany. In 2008 prosecutors reopened investigations after former Italian president Francesco Cossiga suggested that a French missile had shot down the plane by mistake. Cossiga did not explain at the time why he had waited so long before giving his views. Dossiers, books, and even a film called The Rubber Wall have been produced over the years about the mystery-shrouded night of June 27, 1980 when a domestic airliner, belonging to the now-defunct Itavia line, crashed into the sea on its way from Bologna to Palermo. Two international panels examined the wreckage. One concluded the plane had been hit by a missile, while another thought a bomb had been planted aboard the craft in a terrorist act similar to that carried out with even greater loss of life at Bologna train station later that summer. But investigating magistrate Rosario Priore and the prosecutors who have succeeded him insisted they had found clear evidence of flight tracks being tampered with and radar scans cleaned up to remove all trace of other planes in the vicinity of the Itavia jet. Magistrates and victims' relatives think the plane may have become caught in a dogfight between NATO planes and a Libyan jet whose wreckage was found in the southern Italian highlands some months after the Ustica crash. Italy has repeatedly asked NATO, and in particular the US and France, for full cooperation in clearing up the incident. According to reconstructions of the event contained in fictitious accounts, the Libyan jet hid under the Itavia jet and a NATO missile hit the wrong target. The truth may only come out when NATO records are declassified in years to come, many of the theorists say. Senate Speaker Pietro Grasso echoed Napolitano in a message to the association of victims' relatives Thursday. "Many parts of this painful affair are still murky and all the responsibility is still not clear," said the former national anti-mafia prosecutor. The commemoration was marked by further controversy after the state attorney's office said it was weighing an appeal against recent compensation for three Ustica relatives, amounting to 1.2 million euros each. The major government partner, the centre-left Democratic Party, said it was "disconcerted" by the possible move.