Reporter: The search is on, with new solid leads coming in from the oil fields of the west. It could be. I could be paul. Reporter: To southern california. I might be paul fronczak. Reporter: To a... See More
Reporter: The search is on, with new solid leads coming in from the oil fields of the west. It could be. I could be paul. Reporter: To southern california. I might be paul fronczak. Reporter: To a family that added facial hair to the age progression image, and saw the spitting image of dad. I do believe that there is a chance of me being paul fronczak. Reporter: After almost 50 years, there is now new life in the stolen baby investigation that the fbi closed out in the 1960's. We're going to do everything that we can to follow up to see if that baby is out there. The special agent in charge of the fbi office in chicago, bob shields, says the case was re- opened after paul fronczak dna revealed that he was not the stolen baby. Reporter: And he showed me the thousands of page of documents from the old case files. Including one directive signed by then-director j. Edgar hoover. You could tell from the documents, significant investigation with priority. We believe there's someone out there that has information. Reporter: Is there a danger that the person who think she might be the stolen baby would say, "but I don't want to turn in my mother as a kidnapper?" It is. There's always that concern. But I would ask them to reach out to us, talk with us. Reporter: Would you actually prosecute 50 years later? That's the us attorney's office who would make that decision. But we definitely would follow up with them and see whether or not this could be prosecuted. Reporter: Law enforcement experts told us that once the age progression images were made public, we should be prepared for all kinds of people to come forward with their own sad tales. Own reasons why they may be the grown up stolen baby. So far, four separate individuals have come forward, each of them 48 or 49 years old, the right age range. Joel lang and his wife joni contacted us after they saw the age progression images on the abc news website. It's about as close as it's gonna get. It's shocking, the similarities. The hairline, everything is a match. Reporter: Lang volunteered to have his dna tested, telling us that he learned at the age of 8 that he was adopted under what he called suspicious circumstances. Nothing adds up. Nothing is normal. Everything is different all the time and that's you. Reporter: It was much the same story with cody hall of loveland, colorado. His wife pamela contacted us after seeing the story on television. The baby picture really just reminded me so much of cody that it gave me chills. Reporter: Hall said he wanted to take a dna test because he never knew his birth parents, and was told he was just "found somewhere" by a relative. She just said she found me, but I don't know how that was done. Reporter: And then there is david fisher, who lives outside fresno, california. A man with a troubled life that he says grew out of uncertainty about his real identity and parents. Even questioning what appears to be a valid birth certificate. I do believe there is a chance that I am the stolen baby, just due to the fact of all the circumstances of my childhood and how I was raised, and how I was passed around. Reporter: As he provided us with his dna sample, he told us how his wife and children saw the abc news stories. And when they saw the picture pop up, they said, "well, dad, that looks like you." Reporter: And when his family added facial hair to the age progression images, there was a distinct resemblance. I was totally shocked when i saw the resemblance. If I had no mustache, it would be me. Reporter: But we found an even more intriguing lead closer to chicago. In kansas city, where we sent our consultant, former fbi agent brad garrett to meet with a man who did not want to be identified publicly but said he not only looks like the drawing, but that his grandmother was found with an aged clipping of the original kidnap story from 1964 hidden in her closet. There's enough loose ends in this case, in this particular lead, to resolve with a dna test. We can cut to the chase. But with four separate dna samples ready to be tested, the best leads in five decades, our investigation hit an surprising and unexpected roadblock. Paul fronczak reported to us that his parents and his brother refused to provide their dna samples to us, making it impossible for us to see if any of the men were a match. You know, my parents love me and I love them. But for some reason, they don't want to know. And I think by not wanting to know, they don't want to help at all. The fronczak parents, chester and dora, and their other son david say they will work with the fbi. But they declined to speak with us and have not spoken publicly about the case since 1964 when they pleaded for their son's return. It turns out there are a number of questions they have never addressed. Either publicly or with their son. When they brought paul home from new jersey, there was no celebration, and they refused to talk to reporters about what should have been a joyous homecoming. In fact, there was so much uncertainty, that they were required to formally adopt paul as their son. And the previously-sealed adoption files obtained during the course of our investigation, in fact, say the blood tests of the parents and the little boy "were contradictory," meaning the tests did not prove or disprove that the little boy was the fronczaks' son. But we learned, the top detectives on the case, including lt. Jack cartan who led the investigation, were skeptical all along. He was sure that was not the baby. Reporter: Lieutenant cartan's daughter mary hendry says her father said the police and the fbi tried to tell the parents it was not their missing son, but kept it quiet publicly. And the fbi and chicago police, who were working together closely, agreed that they would do that. Reporter: They kind of went along with it? Yeah. I overheard him saying to my mother,"that's not the fronczak baby." Those people are so nice and want to believe it that we're going along with it. Reporter: With the public believing the case had been solved, the fbi and chicago police essentially closed the investigation. The search for the stolen baby was over. And at the fronczak home, paul says his parents never shared whatever doubts they might have had with their adopted son, trying to keep it all a deep, dark secret.
This transcript has been automatically generated and may not be 100% accurate.