Nobody can precisely define this criminal racket with names, goals or structure — not the prosecutors charged with solving Mr. Susini’s murder, the citizens outraged by it or the family members bent on justice.

The rarity of trials makes the organized crime phenomenon all the more ambiguous. The word “mafia” is used freely, but nobody maintains that Corsica is in the grip of an Italian-style centralized structure of organized crime.

Instead, prosecutors say, local gangs and crime bosses have infiltrated government in many parts of the island — four mayors have been killed in 20 years, as have the head of the bar association, the head of Ajaccio’s chamber of commerce and other businessmen. Often, criminals operate under a facade of respectability that makes detection all the more difficult.

Yet many are convinced that the sniper’s killing of Mr. Susini at his beachfront restaurant on the morning of Sept. 12 represents a tipping point.