In its long-running dispute with the police over how to share investigation powers, the prosecution has adamantly opposed police officers being allowed to investigate crimes on their own. One reason is prosecutors’ concern about police officers’ susceptibility to corruption. An unfolding corruption case has proved that this concern is not unfounded.

On Sunday, prosecutors arrested four police officers on charges of having taken 200 million won in bribes from Lee Gyeong-baek, a hostess bar owner once considered the “emperor of the room salon world.” Lee had operated several prosperous hostess bars and illegal casinos in the Gangnam district of southern Seoul until 2010. But he is currently in prison for arranging prostitution and evading taxes.

According to prosecutors, Lee recently confessed that he had provided at least 5 million won a month to the four officers between 2007 and 2010 in return for tips on imminent police crackdowns on hostess bars. At the time, the four suspects all worked for the Department of Women and Juveniles of the Seoul Metropolitan Police Agency, a unit responsible for clamping down on illegal practices at entertainment businesses.

Lee further told prosecutors that the four officers had also taken bribes from other operators of hostess bars in Seoul. He alleged that one of the four had received a total of 5 billion won by collecting between 2 million and 10 million won a month from each of the tens of hostess bars under his influence.

Following Lee’s testimony, investigators searched the suspect’s house and discovered two expensive imported cars and more than 10 bank accounts, which showed regular monthly receipts of large sums of cash.

The imprisoned bar operator also told prosecutors that another of the four officers had received millions of won a month in bribes from a large number of entertainment places, including massage parlors.

To our dismay, prosecutors found that the dirty practice among police agents of receiving a little pocket money from entertainment businesses has evolved into a money-making business. The two police officers were found to have invested part of the money in hostess bars and massage parlors.

Prosecutors’ questioning of Lee was prompted by their suspicion that an earlier police investigation into the case had been far from thorough. The police probed Lee in 2010 amid rumors that many police officers had his back.

The rumors were not groundless. Lee’s cell phone records showed he had frequently talked with at least 66 police officers. Cho Hyun-oh, head of the National Police Headquarters, fired six of them and disciplined 33 others.

But prosecutors have found that the initial police investigation had failed to uncover and punish six more officers who stayed in close touch with Lee. The four suspects arrested on Sunday are among them.

The reopened investigation has further undermined the already shaky credibility of the police. It shows that police officers leaked crackdown information to owners of entertainment places and helped them cut corners in doing business. In return they received large sums of money. This reminds us of organized crime rings that extort money from adult entertainment spots in return for helping them engage in illegal business practices.

Furthermore, the police attempted to cover up the corrupt practices of their officials. Two years ago, they did not indict Lee on bribery charges, saying that they could not find evidence. What they lacked at the time was not evidence but the will to dig up the truth and punish all corrupt officers involved.

Now that the case has been reopened, prosecutors need to go to the bottom. They are advised to look into the possibility of the four suspects offering bribes to their seniors to maintain their lucrative posts. Investigators are also required to broaden their probe to include other officials Lee might have attempted to bribe, such as tax officers, firefighters and prosecutors.

The police must fully cooperate with prosecutors in investigating the case. At the same time, NPH Commissioner Cho needs to find ways to terminate the corrupt practice of police officers protecting entertainment businesses in return for money. Police officers, for their part, are required to clean up their act before demanding more investigative power.