Italian police have searched about 50 homes in the southern village of San Luca in connection with the murder of six Italian men in Germany.
Italian police fear a resurgence of a long-running crime gang feud
Police said they hoped to prevent the bloodshed spreading.
The village in the Calabria region has been the scene of a long-running feud between rival families of the notoriously violent 'Ndrangheta mafia.
Police said the men shot dead early on Wednesday outside an Italian restaurant in Duisburg, were linked to the group.
German police said the victims, aged between 16 and 39, had all been shot in the head.
The San Luca feud began with a brawl in the village on St Valentine's Day (14 February) in 1991 and has since claimed some 15 lives.
"Honoured Society", based in Calabria region of south Italy
Origins thought to date back to shortly after Italian unification in 1861
Group grew as reaction to richer class from north
Operates clan-based power structure based on blood families
Accused of cocaine and weapons smuggling
Estimated to have made 16m euros (£10.8m) profits in 2002
Italian Interior Minister Giuliano Amato said one of the victims was thought to be one of those originally involved in the San Luca dispute.
As well as searching houses in the village, police set up road blocks around the Calabria region.
"We need to deploy units so we can avoid a repetition of these acts," said Francesco Gratteri, the head of Calabria's anti-Mafia unit.
Several Italian detectives have been sent to Duisburg to assist German police who say the link to the San Luca feud is their chief avenue of investigation.
The killings may have been retaliation for the murder last 25 December of the wife of the head of one of the rival 'Ndrangheta families, Italian police said.
Italy's Deputy Interior Minister, Marco Minniti, said the apparent score-settling in a foreign country was "unprecedented" and "an element of great concern".
"This feud marks a second chapter outside the territory where these clans usually operate - this time even outside our national borders," he told a news conference in Rome.