Transcript for Undercover for Alleged Underage Prostitution Raid at Colombia Villa
Operation underground railroad has been on this case for months now and tonight, with the sting about to happen, the danger is real and the stakes are high. A group of Americans hoping to catch pimps trafficking underage girls. Will it work? Once again tonight, here's ABC's David Wright. Hola! Reporter: In Colombia, the bachelor party is on. A parade of pretty young girls ready to work. 30 girls in all. Many of them are very young. The alleged traffickers, true to their word. You're the king, man. Reporter: But this isn't your typical bachelor party. Upstairs, Colombian authorities are quietly waiting for just the right moment to drop the hammer. Tim Ballard plays the part of the rowdy best man. Buying girls for sex. But really, he's in on the sting. He's the leader of a team of Americans who have been setting this up. They go by the name operation underground railroad. And they work with local authorities to catch sex traffickers. Pimps who sell underage girls for sex. Worldwide, over 1 million children are trafficked each year. Many of them sold into sexual slavery. May I ask, your name is Andrea? That's what we're call you. Reporter: Andrea disguised her face for this interview. Her parents have no idea she's a prostitute. She's 17. And she's been working for six years. So, you startled at age 11. This is very young. Do the men pay more for young girls? Especially under 14. How much more? Almost twice the price. Andrea says she wishes she didn't do this. Why do you have to do this now? Reporter: At the operation underground railroad party -- You brought like 100 girls. I'm missing three, they're right. He got 100 girls. 100. Reporter: Some of the girls are just 14 years old. You can tell they're a minor though, right? Of course, of course, absolutely. They're giving eyes out, right? You know, like, trying to flirt with guys. Yeah, so sad. Reporter: Marcus talks up one girl in particular. She's nice. How old? 14. She's here? Yeah, she's beautiful. Reporter: Legally, it's not enough just to have brought them. In order to prosecute, there has to be no doubt why. A clear quid pro quo. She's 14? Yes. Okay, so -- here's the thing. Does she do everything? I'll talk to her. I told her anything crazy that he wants, if she wants more money, she better ask. Not going to be any problems? Reporter: And money has to change hands. In Colombia, it's legal to have consensual sex with a 14-year-old. But paying for sex with a minor is a crime. Money time. This is the good part. These are the two. Okay, let me see. Reporter: Cold, hard cash. Right there on the table. This is the crucial moment investigators need to seal the deal. The 14-year-old -- Do they know what's going on? No, the parents don't know . Reporter: The man on the left there is an undercover investigator for the cti, Colombia's equivalent of the FBI. He gives the code word, agreed in advance. Calling for wine to celebrate. The bust is on. Whoa. What the heck? Reporter: Federal police to arrest the five alleged traffickers. Social workers to attend to the girls. Tim is still playing his part. Who called the cops, man? Reporter: Demanding to see his lawyer as if he's being arrested, too. For us, a chance to talk to Marcus before he's carted away. Is it worth it? Worth what, sir? Reporter: To bring underage girls -- There's no underage girls here. Reporter: You told him there's a girl that's 14 years old. I lied to them about that. Reporter: He tells me that owl all of the girls are of legal age. He insists it's all just a misunderstanding. So, were you lying then or now? I was lying because I wanted him to be happy with the girls that I found, okay, but no, I did not -- there's not a single girl here that's underage, I promise you that. Reporter: Marcus tells me he has a daughter back in the statements who is 15. How could you do this if you have a daughter that's 15 years old? I don't get it. I'm not looking for young girls. Reporter: Do you think this is just a total mistake? Yes. It's a total mistake. You know why, sir? Reporter: Why? I did not look for young girls. Reporter: You brought some. Those are the girls that look young, sir. Please, check their I.D.S. I promise -- Jimmy: I think they' Reporter: I think they will check more than their I.D.S. I hope they do. Reporter: Out by the pum, the social workers are one by one checking I.D.S. No idea. It's okay. Reporter: Many of the girls are afraid they're being arrested. This is their livelihoods. For some of them, their main concern is not to be rescued, but to get paid for a night's work. When they first showed up, some of them looked really terrified. I talked to a few of them and asked them how old they were and some were 12, some were 13, most were 14. One of the girls came and embraced me and started to cry and then I started to cry. It's a very -- Reporter: Your heart goes out to them, huh? For these girls, it's a long road ahead. Over the next few days, social services did interviews with the gi girls, their families and the traffickers. In all, 17 of the girls were under 18. Their I.D.S, fake or nonexistence. What makes me sad is that, yes, we got four traffickers and I hope they fry, but what makes me sad is that I think so many of these little girls, they don't know any different. Reporter: End up back out on the street. Yeah. Reporter: You think they did some good tonight? I know they did. But so much more needs to be done. Reporter: Any underage girl will be taken to a certified foster care program, dedicated to making sure they get the care they need. How do you feel at the end of all this? I mean, you must be elated, but on the other hand -- It's bittersweet. I want to celebrate, but I don't, because it's -- it's so horrific. So horrific. I see my own children in this. Reporter: But anyone of age is free to go. In so many cases, even the youngest girls return to what they know best. The streets. Sure enough, later that night, we saw some of the same girls from the party, the ones who could prove they were over 18, back out on the streets, making up for lost time. Tonight's raid, just an inconvenient ordeal. I'm David Wright for "Nightline" in Cartagena, Colombia. Sad on so many levels. Our thanks tonight to the team in Cartagena for that report. And this update. Operation underground railroad just finished their latest sting this past weekend, freeing, they say, more than 55 underage victims from sexual slavery.
This transcript has been automatically generated and may not be 100% accurate.