Spanish police have arrested 30 people and seized almost 2.5 tonnes of cocaine after breaking up a criminal network that used teams of young swimmers to hide the drugs on moving, Europe-bound ships which were then attacked and relieved of their unwitting cargo before reaching port.

The 15-month investigation began in October 2024 when Policía Nacional officers found 88kg (194lbs) of cocaine in a vehicle in the southern Spanish town of Mijas. The drugs led them to three gangs, including a Balkan cartel, who were working together to bring huge quantities of cocaine into Spain from Colombia.

“The gangs used the so-called ‘monkey’ technique to get the shipments of cocaine into maritime containers that were being transported on container ships,” the force said in a statement on Thursday.

“The drug-trafficking technique involves using youngsters from poor backgrounds who are good swimmers to get the drugs on to ships while they’re at sea. Members of the same organisation then headed to Spain in order to get to the containers by intercepting the ships carrying them before they reached the strait of Gibraltar.”

One such attempt was thwarted in the middle of last year when a ship bound for the port of Cádiz informed the maritime rescue service that it had found stowaways on its deck, leading to the seizure of a container in which 1.4 tonnes of cocaine had been stashed. The stowaways turned out to be three men tasked with recovering the drugs, who then fled.

Not long after, a ship passing through Portuguese waters reported armed stowaways on board. But the men managed to unload bundles of cocaine that had been hidden in a container and hand them to their accomplices before evading the authorities.

In the autumn of last year, officer came across five men – three Colombians and two Spain-based members of the Balkan cartel – who were using speedboats and the “drop-off” method to collect their drugs.

“This method involves throwing the merchandise from a merchant ship for collection by smaller vessels near the destination country, subduing the ship’s crew and extracting the drugs from inside the containers, using military techniques and weapons of war,” the statement added.

The drugs were then hidden in towns and villages along the Gulf of Cádiz and later transported by road to other European countries.

As well as confiscating 2,475kg of cocaine and various assault weapons during the operation, police seized boarding ladders, nautical equipment, eight high-end vehicles, more than €166,000 (£144,000) in cash and watches, and jewellery worth €100,000.

On Monday the Policía Nacional announced its largest-ever seizure of cocaine at sea after officers found almost 10 tonnes of the drug hidden among a cargo of salt on a merchant ship off the Canary Islands.