IIRC, it was a turf war between two Sicilian factions, one of which (the Matranga family) would go on to become Carlos Marcello's outfit. Both sides were trying to control Italian stevedores on the "lemon boats", the police chief threw in with one side and got himself killed for his trouble. After a trial in which the jury was widely believed to have been bought off, a mob stormed the jail and killed (shot, not hanged for the most part) the men thought to be the ringleaders.