Heavy security was imposed for the trial of former Prime Minister Giulio Andreotti after Italy's interior minister said police were worried about a possible Mafia missile attack. A fourth session of Andreotti's trial on Mafia charges got under way with police officers and soldiers ringing the fortified court in Palermo. A helicopter circled overhead and police snipers were perched on rooftops of nearby buildings, witnesses said. Andreotti, 76, who had served as prime minister seven times, is accused of having been the Mafia's protector in Rome's corridors of power. He denies the charges.