Five men were shot dead in less than a minute on 2 January at a bar in a service station outside the provincial town of Vittoria, near Ragusa. Two of those killed were bystanders who had stopped for coffee and found themselves near the targets of the operation - three local bosses of the Mafia.
The killings provoked widespread shock and anger. The Prime Minister, Massimo D'Alema, called an emergency law-and-order summit and the government announced plans to put more resources into Sicily.
Those arrested were mainly members of the Piscopo and Dominante clans, who are fighting for control of the drugs trade and extortion in Vittoria. They included two farmers named Alessandro Piscopo, cousins aged 37 and 39, who are believed to have ordered the revenge attack, and the brother of one of the bosses who was killed.
Investigators in Catania say the killings were the result of a split within the Dominante clan, confirmed by a former local mafia member turned state witness.
"After the slaying we received reports and complaints from local businessmen," one of the prosecutors said.
"It's an important sign because the criminal presence in Vittoria is very heavy and can only be broken if people find the courage to speak out."
The president of the parliamentary Anti-Mafia Commission, Ottaviano Del Turco, praised police and magistrates for their swift action.