Local sources said the ultra boss shot and hurt was Andrea Beretta, 49, while the fan he allegedly stabbed to death was Antonio Bellocco, 39.
The two fans were fighting over a so-far-unknown issue.
Beretta has been arrested on suspicion of murder and is being guarded in a Milan hospital.
The fight broke out outside a boxing gym where the two trained, police said.
Balocco allegedly shot Beretta from inside a Smart car before the leader then allegedly stabbed his assailant in the throat and killed him, despite being wounded in the leg, police said.
Bellocco was a scion of the eponymous ‘Ndrangheta clan from Rosarno near Reggio Calabria.
He was the grandson of the historic clan boss and the son of Giulio Bellocco, who died in prison in January under the tough mafia ‘41 bis’ prison regime.
Antonio Bellocco himself had a record of crime for ‘Ndrangheta, Italy’s biggest, richest and most powerful mafia due to its control of the European cocaine trade.
His arrival about a year ago in ultra leadership circles created friction that probably led to Wednesday’s fight, police said.
ANSA
