Domenico Fontana, 61, and his sons Pasquale, 37, Pietro, 36, Emilio, 32 and Giovanni, 19, died instantly in a hail of bullets in a frenzied attack.
Their bodies were found by Domenico's wife and daughter and although the massacre bore the hallmarks of a mafia vendetta, police said an argument over land and sheep grazing rights was the motive.
The men were gunned down in a farmhouse at Filandari near Vibo Valentia in southern Italy.
Two of the bodies were found outside the building which police said indicated they had been shot as they tried to escape. Yesterday, four people were being questioned in connection with the killings.
A police spokesman said: "The dynamic of the murders would suggest organised crime as being the motive but from what we have established there has been a long-running feud over land and sheep grazing rights between Fontana and his neighbour.''
The men held were named as neighbour Ercole Vangeli, 42, his brother Franco, 54, Ercole's nephew Piero and Ercole's son in law Gianni Mazzitelli.
Forty bullet cases were recovered from the scene and these were traced to two handguns, which were legally owned by Ercole and Franco and two magazine loaders were also recovered.
Local prosecutor Michele Sirgiovanni said: "Vangeli has told us he was fed up of being bullied by the Fontana family.
"They encroached on his land and they were also being intimidated. This resulted in physical violence and had been going on for more than three years.
"This had nothing to do with the mafia - it is similar to blood feuds from the 18th century."