Aug. 8, 2013  -- In 1964 newborn Paul Joseph Fronczak was stolen from the Michael Reese hospital in Chicago by a woman posing as a nurse. She was never caught.

Just over a year later, an abandoned baby found outside a variety store in Newark, New Jersey was believed to be the stolen baby Paul and "reunited" with his Chicago family. But decades later a DNA test now reveal Paul is not the Fronczak's biological child.

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Today, nearly 50 years later, following media attention, the FBI announced they are reopening the cold case of the kidnapped baby.

"We did a cursory review of the file and felt it warranted reopening the case. We will re-interview witnesses and take a good look at physical evidence," said FBI spokeswoman Joan Hyde. "We want to see if the case might be benefited by modern technology not available at the time of the original investigation."

In an interview with ABC News Anchor Barbara Walters, the man who was raised as Paul Fronczak explained how he first learned of the 1964 kidnapping.

"I was snooping through the house like most kids do, around Christmas time, looking for Christmas presents, and I went in the crawlspace and found a bunch of boxes that I thought was going to be the big score, and uh, it turned out it was a bunch of things related to a kidnapping around someone named Paul Joseph Fronczak," he said. "And that was me."

WATCH Barbara Walters' interview with the man known as Paul Fronczak tonight on "World News With Diane Sawyer."

While Fronczak's parents have shied away from media attention, the now 49-year-old Fronczak is determined to discover who he really is, and what happened to his parents' stolen baby.

"My parents raised me, and they did a great job, and I feel that if I don't do everything I can to help find the real child, then I'm not doing my job as a son," he said.

And as the parent of a 4-year-old daughter, he says he is especially eager to discover his own biological heritage for her sake and his own.

"I mean, and to me, in their heart, I was their son, and that's all that mattered," he said. "I have been struggling with the fact that I want to know if the real Fronczak baby is still alive, and what happened. I also want to find out who I am, and why I was abandoned."

ABC News Anchor Barbara Walters and Chief Investigative Reporter Brian Ross are investigating the case and you can send confidential tips to "20/20" by CLICKING HERE.

ABC News' Lauren Putrino, Gail Deutsch and Lydia Noone contributed to this report.