Now questions are being asked whether the authorities recklessly put the men's lives at risk by not stepping in to stop the attacks.
Although the arrest of pimp turned docu-soap star Bert Wollersheim - nicknamed the “Red Light King” - represented something of a coup for Dusseldorf police earlier this month, Police Special Commission Chief Roland Wolff admitted, “It's a wonder that nobody ended up dead.”
The investigators could have been guilty of accessory, facilitation of a crime or failure to render assistance as they watched while the customers were drugged and robbed, criminology professor Christian Pfeiffer told Friday's Express regional newspaper.
The brothel operator, 61-year-old Bert Wollersheim became a household name in Germany after starring in a television docu-soap “The Wollersheims.” He was among 11 men arrested in nationwide raids earlier this month.
Those arrested have been accused of systematically drugging clients with alcohol, cocaine and date rape drugs to gain access to credit cards.
The Dusseldorf-based group is thought to have gained at least €100,000 in this way, although the final figure could be much higher.
“Organised crime must be confronted, even in a covert way, by listening in and using telephone wire taps,” Pfeiffer told the newspaper.
“However, when the structure of a criminal organisation comes to light and there are fears that there might be more victims, then the police should have acted to protect lives,” he said.
“The suggestion that it was fortunate that there were no deaths shows just how dangerous the situation was.”
The victims, clients at four high-class brothels, were always photographed surrounded by prostitutes and bottles of champagne, so the gang could blackmail them later.
The professor said that one vital principal of law-enforcement - that safety should come before the pursuit of justice - appeared to have been ignored.
Express said it had discovered that police and prosecutors had carried out a risk assessment on the danger posed to client's health, as well as their bank accounts.
“Whether they had assessed the nature of the danger in good time and whether they crossed the boundaries of what would normally be deemed acceptable, must be carefully looked at,” said Pfeiffer.
The allegations came to light some two years ago, through a 31-year-old witness known as Christian K. The investigation by undercover officers began 11 months ago, and involved telephone wire taps, bugging cars and analysing bank transaction data.
In all, some 17 alleged victims of drugging were discovered, the drugs having been added to their drinks. A further 40 clients later reported via a telephone hotline that they had been victims.
In the raids, officers uncovered €150,000 in cash, as well as a significant quantity of cocaine, amphetamines and other medical drugs, the Süddeutsche Zeitung reported at the time.
Wollersheim, who has more than 8,000 fans on Facebook, remains under investigation - along with 80 suspected members and associates of his Dusseldorf-based gang.