ROME — It’s well known that ISIS leaders are punishing those who use recreational drugs, which is against their peculiar version of sharia law. But the terror group apparently doesn’t have a problem with pushing those same drugs for profit—which is why it's been teaming up with the Sicilian Mafia to cash in on the lucrative trade.

That’s also why Italy’s chief anti-Mafia prosecutor, Franco Roberti, wants to legalize cannabis and hash, not just in Italy, but throughout Europe. “Decriminalization or even legalization would definitely be a weapon against traffickers, among whom there could be terrorists who make money off of it,” Roberti told Reuters in a wide-ranging interview this week.

The prosecutor, who also heads Italy’s anti-terrorism investigations, has just written a book, The Opposite of Fear, that outlines the similarities and links between organized crime syndicates and terrorist organizations like the so-called Islamic State. He says both use the same criminal playbook. “International terrorism finances itself with criminal activities that are typical of the mafia,” he writes in his book. “Like drug trafficking, smuggling commercial goods, smuggling oil, smuggling archaeological relics and art, kidnapping for ransom, and extortion.”

Roberti has long held that Italy’s fight against organized crime could help lead the battle against ISIS. “The Islamic State is, in effect, a mafia state, it has all the features,” Roberti said when presenting Italy’s National Anti Mafia Directorate annual report in March. “The Islamic State is a transnational mafia organization, which uses external organizations for smuggling [and] drug trafficking—in this case the Mafia.”

He says the Sicilian Mafia, also called Cosa Nostra, has had no choice but to partner with ISIS to continue to use the Libyan coastline for its high-stakes smuggling business. “Certainly [ISIS] controls the Libya route,” Roberti told Reuters. “It controls the coast along the Gulf of Sirte.”

That coastline is the main conduit for the Cosa Nostra to channel drugs into Italy and throughout Europe, an operation that nets the mob some $36 billion a year, according to the United Nations Office on Narcotics and Crime.

According to a report by IHS, ISIS has had to beef up trades like drug trafficking to make up for a 30 percent loss in income as people who would generally pay taxes flee. “The Islamic State has lost about 22 percent of its territory in the past 15 months,” said IHS senior analyst Columb Strack. “Its population has declined from around 9 million to around 6 million. There are fewer people and business activities to tax; the same applies to properties and land to confiscate.”

Libya is not the only country where ISIS deals drugs. As The Daily Beast reported last year, ISIS is also buying up hashish from Lebanon to help fund its battle in Syria. Ahmad Moussalli, a political science professor at the American University in Beirut, says ISIS has taken over significant areas of cannabis crops around the border with Lebanon, allowing them to profit from the trade.

If the lucrative black-market sale of cannabis and hash were decriminalized, Roberti argues that it would cut off a valuable resource of revenue for the terrorists and for the Mafia. “This should be an Italian domestic debate,” he writes in his book. “And also a European one.”