Pawtucket detectives found the ad on backpage.com, complete with photos and advertising sex.
But one of the pictures wasn't a woman. It was of a 14-year-old runaway reported missing from Pawtucket in August.
"We received additional information that her image was depicted on several sex for hire websites," said Maj. Arthur Martins of the Pawtucket Police Department.
Martins said detectives went undercover to find the missing girl.
They traced cell phone numbers listed in the ads and tracked the phones to a neighborhood on the Providence-Cranston line. Then, they set up a sting, meeting 21 year-old Nicole Ferreira.
"This was Ms. Ferreira who was putting this young girl in harm's way. That was our main focus," Martins said.
Court documents say Ferreira admitted to pimping the 14-year-old, telling detectives, "I know she's young ... I just post the info and take the calls."
"The FBI say it's actually a typical age for girls to become victims of sex trafficking. The predators, the traffickers, they know that. They go out looking for places where runaways hang out," said University of Rhode Island professor Donna Hughes, an expert in human trafficking.
Hughes says children working in the sex industry happens much more often that many people realize, and in Southern New England.
She points to the recent discovery of a 15-year-old girl working at Cheaters Gentleman's Club in Providence.
"It is very common everywhere in the United States, but also in Rhode Island. I'm actually really pleased that law enforcement is becoming engaged in doing these investigations," Hughes said.
Ferreira told detectives they could find the missing girl at a house in Cranston.
Inside the home at 125 Westwood Ave. police located the teenager, along with several women, apparently living in the basement.
No one answered the door when NBC 10 visited the home Thursday.
Detectives said the victim told police that "Nicole was the leader, the person that sets up the meetings with men, and the person that takes a portion of the money" and that "she believes Nicole set up about 40 to 50 meetings with men for the purpose of sex."
Police told NBC 10 they're still investigating whether the women found with the girl were also involved with prostitution.
They also say more charges are possible as the investigation continues.