(CNN)One girl wrote the word slave above her genitals. Another was made to bark like a dog while naked.
Cho Joo-bin, a 24-year-old man, hosted online rooms on encrypted messaging app Telegram, where users paid to see young girls perform demeaning sexual acts carried out under coercion, according to South Korean police.
As many as 74 victims were blackmailed by Cho into uploading images onto the group chats, some of the users paid for access, police said. Officials suspect there are about 260,000 participants across Cho's chat rooms.
At least 16 of the girls were minors, according to officials.
On March 20, the authorities announced that Cho had been arrested on charges including producing and distributing child pornography, sexual abuse, blackmail, coercion, fraud, illegal distribution of private information, and sexual assault through illegal filming but he has not been indicted.
For South Koreans, the case has become a lightning rod for a nation grappling with widespread sexual abuse and accusations of pervasive misogyny, after a series of scandals involving the secret or forced taping of women in recent years.
On Monday, President Moon Jae-in responded to the rising anger, calling for police to investigate all members of the chat groups, not just Cho. Currently, 124 people connected to the chat groups have been arrested, and 18 have been detained.
But for many South Koreans, an investigation is not enough.
Over four million people have signed two petitions demanding the heaviest of punishments for the arrested operator and calling for the names and faces of all involved to be released.
Cho, who has only appeared once since his arrest, gave a statement on March 25, at the Jongno Police Station in Seoul before being transferred to the prosecutors' office. "Thank you for stopping a life of a devil which I couldn't stop myself," he said.
Cho's lawyers quit on March 25 according to the law firm and authorities. The law firm told CNN that the reason for this decision was because information they received from Cho's family when they accepted the case was very different from the facts.
It was two university journalism students who brought Cho to the attention of the police last July, officials confirmed to CNN.
The duo, who asked to use the pseudonyms Kwon and Ahn to protect their own safety, discovered the chat groups, which were hosted on the encrypted Telegram messaging app, in the summer of 2019, as part of an assignment to investigate online sexual crimes.
Though the chat groups were private, links to them were easily found by simply using keyword searches on Google.
When Kwon first saw the chat rooms, she said she "couldn't believe the scenes and messages unfolding before her eyes."
"I felt that I needed to bring these serious crimes to the public's attention," Ahn said.
There were many operators running similar chats on Telegram, but Kwon and Ahn focused on Cho, who went by the alias of "Guru" and ran at least eight groups, some of which had over 9,000 members at any one time.
Cho allegedly sourced his victims by posting fake modeling jobs online, police said.
Young women would submit their personal information, including their social security numbers and addresses, so they could get paid, along with photographs.
Once hired, they would allegedly be asked to provide more revealing pictures. Police believe these were then used to blackmail the young women -- Cho allegedly threatened to post them on the internet with their personal details if the girls did not work in his Telegram chat rooms, police said.