. >> reporter: one of those cops, this man, detective bill guyer. >> i remember meeting the detective. almost reminded me of my dad. >> reporter: he formed a special bond with natalie. he helped her through the trial. hobson is sentenced to 26 years for the commercial sex abuse of a minor. >> i tell her and every other girl, i'd like to get them to the point where they can't wait to get on the stand and say that's the person that did it to me. >> reporter: they've agreed to take us inside a back page sting. >> we want to post an ad and do it under escorts. >> reporter: this detective frequently goes undercover. he's asked we not show his full face. >> i am 18 years of age or older. you just say i agree and continue. one click and you're three. >> reporter: we're here to see what happens when you post a back page ad that suggestions an underage girl. this is for an 18-year-old girl with the mention of a younger frenld. ask about my younger friend. the ad is up and running. in minutes we have an ad advertising an underage girl. we've gotten several phone calls, several texts. >> another