Investigator Renato Cortese said three of the suspects were already in custody and a fourth was arrested early Friday in Reggio Calabria in the southern region of Calabria.

Authorities say the explosion before dawn Aug. 26 was the latest act of intimidation by the ‘ndrangheta, a global cocaine trafficker. It shattered windows and the door, but magistrate Salvatore Di Landro and other occupants were unharmed.

Cortese said information provided by boss Antonino Lo Giudice, who turned state’s evidence, helped lead to the arrests.