"Decriminalization or even legalization would definitely be a weapon against traffickers, among whom there could be terrorists who make money off of it," Roberti said.
He said Italy's mafia makes an estimated $36 billion annually from the illegal drug trade; narcotics accounts for almost 10 percent of the Islamic State's funding, according to a report cited by Reuters.
"International terrorism finances itself with criminal activities that are typical of the mafia, like drug trafficking, smuggling commercial goods, smuggling oil, smuggling archaeological relics and art, kidnapping for ransom and extortion," Roberti said.
Italian authorities have been investigating suspected links between the proxies of terrorist groups and Italian mafia families, which oversee a variety of criminal activity — drugs, gun-smuggling, forgery of identity papers.
“Naples has been, for many years, a central logistics base for the Middle East. The Camorra is also active in the world of jihadist terrorism that passes through Naples,” Roberti told the Daily Beast last month, referring to the main mafia organization in the Italian city. "Naples lends itself to this type of activity. In the past there have been contacts between jihadi militants and the Camorra clans."
In the face of this complicated and shadowy threat, a wider war on drugs seems counterproductive.
"We spend a lot of resources uselessly. We have not succeeded in reducing cannabinoid trafficking. On the contrary, it's increasing," Roberti told Reuters. "Is it worth using investigative energy to fight street sales of soft drugs?"
Advocates of marijuana legalization and other drug policy reforms have made this sort of argument for years. But there are signs that it is now gaining wider traction.
"Focused overwhelmingly on criminalization and punishment, [existing anti-drug policies] created a vast illicit market that has enriched criminal organizations, corrupted governments, triggered explosive violence, distorted economic markets and undermined basic moral values," the letter says.