To everyone's horror, the police discovered that the containers were home to various prisons, as well as a soundproof dentist-style torture chamber.
Dutch authorities raided the torture chamber last month, before they had the chance to be used.
This video released by Politie Landelijke Eenheid shows the police operation as they enter the building in Wouwse Plantage and make the discovery of the chairs, chemical toilet and police clothing, opening the containers one-by-one.
Seven shipping containers were found in a warehouse on June 22 in the town of Wouwse Plantage near the southern border with Belgium.
The suspects called the containers their "EBI" - a reference to the Extra Beveiligde Inrichting, a Dutch secure prison.
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The grisly discovery gave a chilling insight into the increasingly violent Dutch criminal underworld, which is involved in the large scale production and trafficking of drugs.
Police had been monitoring communications on an encrypted network called EncroChat since April, where suspects talked freely about their activities, according to a police report.
Dutch authorities said last week that their investigation, codenamed 26Lemont, based on millions of messages from the EncroChat phones, had led to the arrest of more than 100 suspects and the seizure of more than 8,000 kilograms of cocaine and 1,200 kilograms of crystal meth, as well as the dismantling of 19 synthetic drug labs and the seizure of dozens of firearms.
The chats also helped the police figure out who the intended victims were, who have now been informed and have gone into hiding, officers said.
"Six served as prison cells in which people could have been tied up and one other container had the sole goal of being a torture chamber", said the Dutch police's central investigations division head, Mr Andy Kraag.
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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.
When they believed that the containers were nearly finished, they raided 13 locations on June 22 and arrested the six men.
Police were able to read texts sent on the encrypted messaging app EncroChat, enabling them to follow real time conversations between the criminals, who swapped pictures of the dental chair, the statement said.
In searches of other properties, including what police described as a base for the criminals near the port city of Rotterdam, officers found police uniforms and body armor, stolen vehicles, 25 firearms and drugs.
A court in Amsterdam ordered the six suspects to be held for 90 days as investigations continue.
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"This is a great result of the 26Lemont investigation", Mr Kraag said.