PALERMO, Italy — The investigators for Italy's antidrug unit were used to measuring the flow of hashish from Moroccan fields to European shores one speedboat or Jet Ski at a time.
So when the phone rang with a tip that an enormous freighter loaded with hashish was plying international waters south of Sicily — bound for Libya, hundreds of miles to the east of the usual quick drug route to Spain — Francesco Amico, a senior investigator, immediately knew something odd was going on.
Not just odd, but huge: When two Italian navy warships eventually stopped the freighter, the Adam, off the Libyan coast on April 12, 2013, agents found a terrified Syrian crew and 15 metric tons of hashish — a stash many times larger than Italian officials had ever seen.
"There was so much of the drug that we didn't know where to put it," said Amico, who waited in the Sicilian port of Trapani for the escorted ship to arrive. "We had to go out and rent a warehouse."
The Italian officials had stumbled onto a lucrative new trafficking route that stretched far to the east along the coast of Northern Africa — and always led to Libya, in an area fought over by competing armed groups that included the Islamic State.
The Adam was the first of 20 ships that would be intercepted over the route over the next 32 months, officials say. Their collective cargo amounted to more than 280 tons of hashish valued at $3.2 billion — roughly half of what was seized in all of continental Europe last year, according to the European Union's Monitoring Center for Drugs and Drug Addiction.
Then the tips dried up, and the busts stopped. The Italian investigation, which had expanded to include other European countries and the U.S. Drug Enforcement Administration, has not seized any ships on the route in 2016, though officials say they believe the traffic continues.
Instead, as they have sought to understand what happens to the enormous shipments, they are struggling with a mystery that has prompted intriguing questions but offered few answers.
One thing they know is that the drugs were not ending up in Libya. The Moroccan drug producers consistently use individualized branding logos, like a scorpion or a dollar sign. That helped investigators pick up the drug shipments' trail again after they left Libya, traveling along an overland route through Egypt and then on to Europe through the Balkans.
But the investigators are still not sure what happened as the drugs passed through. From interrogations and surveillance, they know the route crossed territory that until a few weeks ago was claimed by the Islamic State — which has taxed shipments of drugs and other goods in Syria and Iraq.
That, in particular, led the Italian drug investigators to start asking questions they never expected to confront: Could the Islamic State or some other group be profiting from the drug route by taxing it? Was the militant-driven chaos in Libya providing an opportunity to drug traffickers to pick a route the authorities wouldn't suspect, or were the Libyan-based groups more directly involved? The investigation continues.
"Once it reaches Libya, we lose track of it," said Lt. Col. Giuseppe Campobasso, who heads the antidrug unit in Sicily of the Guardia di Finanza, the Italian financial police force that led the search for the ships.
For years, the Italian investigators had tracked small shipments of Moroccan hashish, around 100 kilograms at a time, coming to Spain in boats that crossed the Strait of Gibraltar — a narrow passage that ferries cross in under 35 minutes. In 2007, Spain began installing cameras up and down its southern coastline, but at least at first, the hashish traffic continued in the usual way.
With Europe's eyes trained on small-boat traffic coming from the south, at first no one noticed the cargo ships that were taking a big dogleg to the east.
Giacomo Catania, an inspector with the Guardia di Finanza who was in charge of storing the incoming drugs, described another oddity: The enormous cargo ships they seized — some as long as a soccer field and designed to carry fleets of automobiles or cargo containers — were empty except for the drugs.
"These are ships that have a capacity to carry thousands of tons, and the cargo in most cases was around 20 tons. Only a minuscule portion of it was used," Catania said.
That the smugglers were willing to operate so inefficiently — Catania compared it to using an 18-wheeler to transport a single pack of cigarettes — is testament to the value of the cargo.
Considering that hashish sells for $11,400 per kilogram once it reaches Europe's streets, the Adam's cargo alone was worth at least $171 million. And shipments seized later were even bigger — including a load of hashish aboard the freighter Aberdeen, boarded in the summer of 2014, that was estimated to be worth $480 million.
After seizing the Adam, investigators in Italy interrogated its crew members, who insisted that they did not know that hashish was in the 591 plastic bags investigators had found on the ship's deck.
The ship's captain testified that he believed he was transporting humanitarian aid, brought to the ship by the crew of a speedboat that approached them off the coast of Morocco and insisted that they take the bags, according to a transcript of his statement to investigators.
To learn more, the investigators — who have decades of experience dealing with the Sicilian Mafia — decided to bug the cells where the six crew members were imprisoned. Over months of surveillance, Amico began to discern the outlines of the trafficking route and the mystery of the militant stronghold in coastal Libya it passed through.
Since the ouster of the Libyan leader Moammar Gadhafi in 2011, stretches of Libya along the coastline in the eastern region of Cyrenaica had become a battleground for competing militant groups. By 2014, that included the Islamic State's branch in Libya, which at various times had footholds in the cities of Benghazi, Derna and especially Sirte, which has partly fallen to pro-government forces.
Italian officials believe those cities were all destinations for some of the drug ships, although the navigation units of other seized vessels indicate they were headed to the Libyan port of Tobruk, which is controlled by a rebel group fighting the Islamic State.
Investigators say they believe that at least in some cases, the terror group would have been able to exact a tax in return for the drugs' passage. That matches the Islamic State's business practice in its stronghold in Syria and Iraq, where, according to one study by IHS Country Risk, 7 percent of the group's revenue last year was from the production, taxation and trafficking of drugs.
But officials concede they cannot be certain what role, if any, the Islamic State might play in the hashish shipments.
"No one has eyes on the ground to say that they know for a fact," said Masood Karimipour, the United Nations Office on Drugs and Crime's regional representative for the Middle East and North Africa. "What we can offer, or make reasonable inferences from, is that where the terrorists are holding terrain, they are controlling everything that goes through it, including the trafficking of whatever — whether it's weapons or drugs."
Poring over the surveillance tapes of the jailed crew of the freighter Adam, Amico and the rest of his investigative team started to wonder whether the drugs were part of a bigger scheme, perhaps involving weapons.
They learned that the Adam had begun its Mediterranean voyage in Cyprus, where it picked up four containers of "furniture," destined for Benghazi. After dropping off the cargo, the vessel continued to Morocco, picking up 15 tons of hashish and returning to Libya.
From other comments by the crew, Amico and his team came to believe that the "furniture" was a shipment of weapons, he said, an idea supported by two of the prosecutors involved in the case.
"Libya is not a land where people consume hashish," said Deputy Prosecutor Maurizio Agnello. "So that cargo of drugs is definitely a method of payment, a kind of coin."
In recent months, however, the Italian investigation has stalled. Most of the tips were coming from French officials, who in recent months were scrambling to deal with a wave of terrorist attacks at home, and when the tips ended, so did the busts. No ships have been intercepted along the route in 2016, though investigators say they believe the path is still being used.
They expressed unease about the lack of certainty regarding the players controlling the new route.
"If this was controlled by the Mafia, we would know how to deal with it, because we know the Mafia well," said Agnello, whose office is in a stately building in Palermo, a stronghold of the Sicilian Mafia, which for years controlled the hashish arriving from Morocco via Spain.
When it comes to the possible involvement of terrorist groups, he said, "It frightens us because they don't have limits. They do stuff that would be unthinkable for a Mafioso."