In a statement on Tuesday, the National Prosecutor's Office said the seven shipping containers were discovered during a raid last month in a Wouwse Plantage warehouse near the border with Belgium.
The shipping containers were lined with sound-proofing material and heat-insulating foil with handcuffs dangling from the ceiling and attached to the floor. Each was equipped with a camera mounted in one of its corners encompassing a clear view of the entire cell.
The container the suspects called "the treatment room" contained a dental chair with straps to lash one's arms to its armrests and handcuffs at its footrest. Bags were also found in the so-called torture chamber, the statement said, containing "pruning shears, loppers, branch saws, scalpels, pliers, extra handcuffs, finger cuffs, tape, balaclavas and black cotton bags that can be pulled over the head."
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In one container, agents also discovered several sets of law enforcement clothing, bulletproof vests and flashlights.
Three stolen vans, two BMWs, seven small guns and a Chinese version of the Russian Kalashnikov AK-47 assault rifle were found in the warehouse, which also contained a sitting and sleeping area believed to be made for guards, police said.
Police arrested the six men on June 22 following an investigation into a 40-year-old man from the Hague suspected of being involved in drug trafficking.