Inside the containers, police found pruning shears, loppers, scalpels, piers, handcuffs, tape, balaclavas and cotton bags that can be pulled over the head.
Dutch police have arrested six men after discovering sea containers converted into a prison and a "torture chamber."
Police in the Netherlands said they discovered the seven containers - one of which was complete with scalpels and handcuffs anchored to the ceiling and floor - before they could be used and that the intended victims have been alerted and gone into hiding.
Authorities found the containers, located in Wouwse Plantage, some 65 kms south of Rotterdam, after launching an investigation in April into a 40-year-old man from the Hague suspected of being involved in drug trafficking.
It came after French police cracked data from encrypted phones used by criminals which enabled law enforcement in the Netherlands to read messages live.
The correspondence between criminals included photos of a sea container "with a dental chair, with straps on the armrests and footrest" as well as "talks of kidnapping and torture," Dutch police said in a statement.
Officers put the area under surveillance from mid-April and saw a few men working in the sea containers almost every day.
When they believed that the containers were almost finished, they raided 13 locations on June 22 and arrested the six men. They are being held on suspicion of preparing kidnappings, serious abuse, extortion, and participation in a criminal organisation. Two of them have also been charged with possession of weapons.
Six of the containers had been converted into a makeshift prison cell with the last one intended as a torture chamber.
Inside the containers — which had been finished with sound-insulating plates and heat-insulating foil — police found pruning shears, loppers, scalpels, piers, handcuffs, tape, balaclavas and cotton bags that can be pulled over the head. They also found several sets of police clothing, bulletproof vests and flashing lights.
From the other locations, officers recovered 24kg of MDMA, stolen vans and cars, and 25 weapons including an automatic assault rifle and seven guns.
Dutch police said last week that the joint Franco-Dutch investigation into EncroChat — a crypto communication service used by criminal networks — had so far led to the arrest of 100 suspects and the seizure of 8,000kg of cocaine and more than 1,200kg of crystal meth.
During the investigation, codenamed 26Lemont, officers read 20 million messages live, the content of which "far exceeded our imagination," police said.
Law enforcement were prevented from reading live messages from June 13 after EncroChat sent a push notification to its users advising them to throw away their phones immediately suspecting they'd been infiltrated by a government agency.
Andy Kraag, the head of the National Criminal Investigation Service stressed, however: "We now know better how they work: a true game-changer for investigations."