Chicago authorities say nine people arrested on sex trafficking charges enslaved girls as young as 12.

Cook County State’s Attorney Anita Alvarez on Wednesday announced the results of a long-term undercover investigation. The investigation was conducted under the provisions of the Illinois Safe Children’s Act, which was signed into law last year. It allows for the use of wire taps, which experts say is a new practice for states.

Lynn Johnson is with the Chicago Alliance Against Sexual Exploitation. The organization helped draft the legislation. Johnson said the law also makes a shift treating minors as victims instead of criminals.

“The way things have been for a long time is that law enforcement would enforce prostitution offenses by arresting and re-arresting the women and girls who are being prostituted. If we don’t hold the pimps and traffickers and people who buy sex accountable, it means the industry thrives,” she said.

Four of the defendants appeared in court Wednesday and were ordered held on bonds up to $1 million. The other five are scheduled to appear in court Thursday. Alvarez says dozens of girls and young women were intimidated and beaten into prostitution.

Experts estimate at least 16,000 girls and women are involved in Chicago’s commercial sex trade.