(ANSA) - Rome, December 17 - Senate Speaker and former chief anti-mafia prosecutor Pietro Grasso questioned government plans Tuesday to make prison conditions tougher on convicted mob bosses. "Honestly I don't know what else can be done to 41-bis," said Grasso. The government's 41-bis law covers Italy's most dangerous criminals and can keep them imprisoned in single-person maximum-security cells, almost entirely cut off from the world to prevent further criminal activity. "I feel like the most one could do has already been accomplished," he added. On Monday Interior Minister and Deputy Premier Angelino Alfano said the government was ready to increase prison security after Cosa Nostra "boss of bosses" Toto' Riina issued further threats against Palermo Deputy Prosecutor Nino Di Matteo from behind bars in November.