(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.