PARIS (AP) — Anti-organized crime specialist investigators probing a wave of attacks on prisons and prison staff in France are looking at the possible involvement of a notorious drug cartel, the Paris prosecutor said Saturday.

The so-called DZ Mafia is suspected of being one of the main narco-trafficking networks working out of the southern French port city of Marseille, which has a long history as a hub for the drug trade and banditry associated with it.

Paris prosecutor Laure Beccuau said at a news conference Saturday that one of the suspected organizers of last month’s prison attacks claims to have ties with the DZ Mafia. She said police investigators will examine “the real or supposed influence of the DZ Mafia” in the violence.

Several prisons were targeted by gunfire and arson, including attacks on prison workers’ homes and on cars at a prison service school, in the Paris area and elsewhere, the prosecutor said.

The letters “DDPF” were graffitied on some targets, believed to stand for “défense des prisonniers français,” which translates as “defense of French prisoners.”

The Paris prosecutor said 21 people detained for suspected involvement in the violence have been handed preliminary charges for attempted murder and other alleged crimes.

French authorities in recent months have stepped up policing against drug trafficking, concerned about growing cocaine use in France and violence associated with the trafficking of that and other drugs.