Updated List

September 20, 1943 - Vito Cascio Ferro died of hunger because the prison guards left him in the cell after the prison was evacuted due the american invasion of Sicily;

January 5,1945 - Peter LaTempa was poisoned on Vito Genovese order because agreed to testify against him;

October 28, 1974 - Richard "Richie" Bilello,mob associate and informant who served under capo Paul Vario,killed by Donald Frankos on lucchese order;

October 28,1975 - Angelo La Barbera a mobster that started the first mafia war in the 1960s was stabbed to death in Perugia prison by 3 mobsters.

August 26,1976 - Domenico Tripodo ndrangheta leader in Reggio Calabria was killed in Poggioreale prison in Naples by Pasquale Barra as NCO favour to the De Stefano ndrina that after Tripodo death menaged to rule on the whole Reggio Calabria;

July 1977 - Vincent Papa (Lucchese) stabbed to death by the aryan brotherhood because was an informant;

January 31,1980 - Antonino Cuomo capozona of Castellammare di Stabia for the NCO was killed in prison on Cutolo's order;

August 17,1981 Francis Turatello,a powerful gangboss was killed in Sardinia prison by Pasquale Barra;

September 1, 1981 - Francesco Diana, socialist councilor of San Cipriano di Aversa, riddled with thirty-five stab wounds in the Aversa prison on Cutolo'sorder;

June 16, 1982, Alfio Ferlito was killed in an ambush when he was escorted by the Carabinieri during a transfer between two prisons. Also three agents and the driver were killed. The hit was ordered by his enemy Nitto Santapaola;

May 11, 1989, Vincenzo Puccio was beaten to death in his cell at the Ucciardone Prison in Palermo, by fellow inmates Antonino and Giuseppe Marchese, two other Mafiosi who had been acting on Riina's orders. Puccio had been planning on overthrowing Riina and Bernardo Provenzano

July 8, 2013 – Powerful Montreal mob figure and Rizzuto rival, Giuseppe (Ponytail) De Vito, is poisoned to death in his cell in a Quebec prison.

Thanks Hollander.