Transcript for Dad Leads International Search for Abducted Sons
We continue with "On the run." Here again, David Muir. Reporter: Somewhere in Europe a mother and her two sons on the run. And we're right there on the trail. "20/20" following the leads. The tips. Two fathers are convinced will bring back their boys. Maria Pfeifer has taken her two sons on a summer vacation to her native Slovakia, defying her two ex-husbands and the courts, she does not return. Her last e-mail to one of the dads, sent just hours after they were supposed to land back in America, saying "I had to reschedule. The youngest boy was sick. Thank you for understanding." She went dark on her phones, went dark on e-mail. We couldn't locate them or find them. Reporter: Authorities say what maria has done is sadly growing increasingly common. Just this week, the state department revealing that 1,000 children have been taken this way. But, this case is unique, not only because maria has the two boys from two different dads, but she didn't return to her home country to simply resume her old life she simply disappeared, authorities are convinced this is a mother on the run. T's tragic enough to take a child away from one of the parents, it's another thing to live a fugitive life with a child. Reporter: With maria failing to return from that vacation as promised, the Los Angeles courts now award full custody of the boys to their fathers. And both men get to work. The world is too small a place. Everyone leaves a footprint. Reporter: But the weeks quickly turn into months. Maria remains one step ahead. Changing e-mail addresses. Eluding a bounty hunter hired by Larry to try to find the boys. It didn't work. We didn't know what we were doing, and we just kind of threw a lot of money at it. Reporter: The other dad, bob, wants far more than a bounty hunter, he wants the law on his side. His friend Stacey Dutton helps him. He devoted every single waking moment to trying to get his kid back. Reporter: He spends months pressing for criminal charges. Meeting with the Los Angeles police and the D.A. And he's about to turn to one more place. We had no money left, had no leads. So the idea came up to go publicand use social media. Reporter: He decides to build an army from the very kitchen where he used to make jerry breakfast. Facebook. Twitter. All of it. To bring jerry home. I helped him build the Facebook page and build, little "Jerry's army." We have 12,110 people on it. Reporter: E-mails begin pouring in. The mother no longer in Slovakia, but now hopscotching around Europe, she'd left possible traces in Prague, in Munich. Then the muscle behind the movement bob is hoping for, the L.A. D.a. Pressing charges against maria. He now knows if he finally gets a solid tip he has the charges to go after her. She's charged now. Reporter: I mean, this is a big deal. That's huge. I prayed for a year. Reporter: Right around then, that maria might be sending e-mails from right here in Paris. And that tip came in from one of her own friends in Los Angeles. And based on that new tip, bob tells us that after all of these months he can still hear his son's voice. He was about to use his own. He heads to Paris and without an appointment walks straight into the U.S embassy there demanding a meeting with special agent Eugene Casey with the FBI. Had you ever seen a case like this before, a dad? No. Reporter: Who shows up at the Paris embassy? Never. Reporter: The agent is upfront about the years that often go by when children are stolen, taken overseas. When bob asked you, what are the chances I'm going to get my son back? You had to be honest with him? Yeah, I told bob she could have told them you're dead. Reporter: You warned bob that these kids, their names could be different, their appearances could be different? Yes. Reporter: But, despite the odds, agent Casey immediately begins chasing the leads from that dad. Tips from perfect strangers all over the world and that one clue that she could be e-mailing from Paris. So, you alert the fugitive squad to hunt down this maria Pfeifer just outside Paris? That's correct. Reporter: And they were on their way? Yes. Reporter: They find an address. An apartment building with a maria Pfeiffer living in it. Apartment 129. Maria Pfeifer. She points us to the next building over, where we find her name on the door. We ask neighbors if they'd ever seen that beautiful young mother and her two blonde boys. Do you know her? No. Reporter: Everyone telling us what they told investigators. That it was the wrong maria Pfeifer. The lead is a bust. Now back in California, bob is back online. And what he does next changes everything. Concentrating on maria's home country now, Slovakia. Posting images of maria and the boys everywhere. And literally post it on every Facebook page in the target area. Reporter: Reporters in Slovakia get word of this and soon start asking the questions bob's been asking for 17 months now. The headlines now everywhere. The blonde maria and her boys missing for more than a year when suddenly the biggest break in the case. The fugitive mother and her boys spotted. Look at the video of maria and her sons. She's right there in the Orange shorts out in the open at a resort in Slovakia. And what's most startling, take a closer look at the boys. Both of them now have long hair. The youngest, Sasha wearing pigtails. Complete strangers sending these clips to bob after hearing about the mother on the run. And that's Sasha, to the left? That's Sasha, in pigtails, disguised as a girl, in the yellow shirt. Reporter: And jerry is coming around right here, right? Jerry. Jerry is in the -- with the long hair. Reporter: How strange is it to see your son disguised as a little girl, halfway around the world? It's surreal. Reporter: And just as special agent Casey had predicted to bob before, another clue comes maria has given the boys new names. The sources telling bob that jerry is no longer jerry. Now "Milos." Sasha is now "Elie." And to hide her movements, maria is no longer using credit cards. She is using a lot of cash. Reporter: Where is this cash coming from? That's the million-dollar question. If we could follow the cash, we would have her. Reporter: Bob turns the stunning images over to the FBI. The international dragnet is now tightening. And one more crucial clue is about to emerge. I'm waiting right now for the word from law enforcement to get on a plane and go to Europe. Reporter: You hope that call comes? I dream of and pray that call
This transcript has been automatically generated and may not be 100% accurate.