Bangladesh, July 30 -- Frances authorities are investigating a string of violent incidents that have heightened concerns over the growing influence of organized crime beyond the southern port city of Marseille, after alleged threats against a French mayor and an extortion-linked arson attack drew national attention. At the center of the investigations is the notorious “DZ Mafia,” a Franco-Algerian criminal syndicate widely associated with the escalating drug trade in southern France. While the group has long been blamed for deadly turf wars in Marseille, it has now taken the unusual step of publicly denying responsibility for some of the latest incidents.

Over a period of roughly 10 days in July, two high-profile cases-one in...