Italy's interior minister was traveling to a southern city a day after four people were gunned down in a mafia killing that has focused attention on an underestimated crime network in the Puglia region.

Authorities say a local crime boss was the main target when killers traveling in a car opened fire on another vehicle in Foggia, killing the boss and his driver. The killers then chased down another vehicle with two farmers who happened to witness the hit, killing them.

The ambush brought to 17 the number of mafia murders this year in the Foggia province, the spur of the Italian boot.

Top anti-mafia prosecutor Franco Roberti told RAI state radio Thursday that "this very ephemeral form of criminality in Foggia has been too long considered a second-class mafia."