The six men killed in the German town of Duisburg last August were all linked to the Pelle-Vottari clan, sworn enemies of the Nirta-Strangios.
About 30 people have been arrested since the German attack.
Locals say the San Luca feud started with an egg-throwing incident at a carnival in 1991 and escalated after Nirta's sister-in-law Maria was shot dead on Christmas Day 2006, a murder which broke the 'Ndrangheta's code of honour.
Investigators see the feud as a fight for control over the drugs trade.
The 'Ndrangheta control much of the cocaine trade in Europe and make an estimated $68bn a year from their rackets, equivalent to three per cent of Italy's economy.
Police say Nirta became acting head of the clan when his father and brother were arrested after the Duisberg attack.
Officers suspect Giovanni Strangio, his fugitive brother-in-law, of direct involvement in the German killings.