The ’Ndrangheta from Calabria has long spread its tentacles beyond Italy, but it has always kept a low profile. The violence at Duisburg is a rare and disturbing instance of its bloody feuds breaking out on foreign soil.

The organisation gets its name from a Greek word meaning “honour” or “virtue” and is less well-known internationally than the Cosa Nostra (Sicilian Mafia) or the Neapolitan Camorra.

Until recently this closely knit web of about 150 Mafia families confined its activities — mostly kidnappings — to rural southern Italy. But Italian anti-Mafia police say that it is now one of the most powerful criminal organisations in the world.

Eighty per cent of Europe’s cocaine passes through the Calabrian port of Gioia Tauro and the clans’ operations have also spread to money-laundering and extortion, prostitution and arms trafficking.

Alarm bells began to ring in Italy earlier this year when authorities in Milan voiced concern over the “infiltration” of the Milanese economy by men with Calabrian accents. When a local mayor near Milan excluded Calabrian-run building companies from public contracts he received a “Happy Easter” card with a bullet inside it and his car was set on fire.

Milan prosecutors say that the ’Ndrangheta has taken control both of illegal drugs trafficking — mostly cocaine — and legitimate businesses in the north, and together with its international links has a turnover of €40 billion (£27 billion) a year, equivalent to 3.5 per cent of Italian GDP.

Laura Barbaini, the Milan prosecutor, said: “The ’Ndrangheta controls banks, restaurants, shopping centres, construction companies, betting shops, luxury boutiques, supermarkets, nightclubs, discotheques and gaming arcades.”

The group is said to have “swapped cloth caps for city suits” and is estimated to have 10,000 “members” in Italy and abroad, largely Calabrian emigres. They retain their “blood ties”, often using local dialect as code. The ’Ndrangheta has branches in Germany, Spain, the Netherlands, South America, the US, Australia and Canada. Canadian police have identified a powerful ’Ndrangheta operation in Ontario, dubbed “the Siderno group” because its members come from that town.

It has gained powerful behind-the-scenes political influence in Reggio Calabria, its original base. Two years ago, Franco Fortugno, the anti-Mafia deputy chairman of the regional Parliament, was gunned down as he voted in the town of Locri. The governor of Calabria, Agazio Loiero, said afterwards that the ’Ndrangheta had become “even stronger and more dangerous” than the Sicilian Mafia.

Marco Minniti, the deputy Interior Minister, said that the Duisburg shootings were part of a feud between the Nirta-Strangio clan and the Pelle-Vottari clan, both from San Luca in Calabria. The feud is believed to date back 1991 and involve rivalry over international drugs and arms trafficking, extortion and other crimes. Mr Minniti said that the clans had deliberately taken their feud to German soil, and that the killers had set out from Calabria.

Luigi De Sena, the deputy head of the Italian police and former police chief at Reggio Calabria, said: “This score-settling is unprecedented. People from Calabria have a very strong presence in Germany but so far they had kept a low profile, trying not to attract attention.”

Antonio Manganelli, the head of the Italian police, said that he had sent a team to Reggio Calabria and San Luca to investigate the shootings and to try and prevent revenge killings.

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