Story highlights "I believe that we will solve these 2 tragic mysteries," Fronczak texts CNN

FBI pledges "a thorough and complete investigation" into 1964 baby kidnapping

Paul Joseph Fronczak, now 49, tells KLAS he wasn't the kidnapped Chicago baby

Fronczak wonders "how old I am, or who I am, or what nationality"

A 49-year-old Nevada man grew up believing he was kidnapped from the hospital as a day-old baby and was returned at age 2 to his parents in Chicago, months after being found abandoned in a stroller in New Jersey.

The FBI closed the case: The toddler looked like the missing newborn.

The audacious kidnapping -- by a woman dressing as nurse who told the mother to hand over her infant because the doctor needed to examine him -- was a sensational crime nearly a half-century ago.

But now the life of Paul Joseph Fronczak has taken another extraordinary turn, he told CNN affiliate KLAS.

After taking a home DNA test, Fronczak said, he discovered he wasn't the missing baby.

The FBI has reopened the case to tackle the questions of what happened to the real kidnapped baby and who Fronczak is, exactly, he told KLAS

"I don't know how old I am, or who I am, or what nationality, all those things you just take for granted," he said. "The FBI decided that because my ears matched the Fronczak baby that I was probably the Fronczak baby."

On Thursday, Fronczak told CNN he's optimistic about a resolution.

JUST WATCHED Kidnapping's happy ending was false Replay More Videos ... MUST WATCH Kidnapping's happy ending was false 04:05

Wrote Fronczak: "I believe that we will solve these 2 tragic mysteries. I feel we are one step closer to solving this, and one step closer to a happy ending!"

He declined to comment further.

He told the Las Vegas station the furor surrounding the 1964 investigation "was huge."

"My parents had letters from the pope, letters from people all over the country. It was a huge case," he said. "My parents got really frustrated because they had reporters hanging outside their windows, climbing telephone poles, taking pictures of them at church, following them all over the place."

Federal investigators in Chicago hope modern technology will help solve the 1964 case, said FBI special agent Joan Hyde. The FBI has a destruction schedule for old files, and agents were happy that the case file wasn't on that schedule, she said.

"We weren't sure if it was still around, but we were pleased to find it did exist," Hyde told CNN.

Hyde declined to comment when asked if the agency felt any embarrassment about the reversal in the case. No one was ever charged in the 1964 kidnapping.

"They conducted a thorough and complete investigation when it originally occurred, and we'll do that again," Hyde said. "We now have technology and tests that we didn't have then, and we're hoping this time around we'll get that piece of information that will help us solve this case, either from evidence or witnesses."

Fronczak's parents, who are still alive, are cooperating with the FBI, but agents are concerned whether their DNA would yield any matches in DNA databases, Hyde said.

"If we take hair or fiber or DNA from the victim's side of the crime, we have to get to the other side of it to compare it to -- to find the persons who did the kidnapping," Hyde said.

Maybe new witnesses will come forward almost a half century later, Hyde added.

With the FBI's mandatory retirement age of 57 for agents, the original Chicago FBI investigators would no longer be with the agency, she said.

Today's investigators will concentrate on who was the kidnapper -- and not on what is Fronczak's real identity, Hyde said.

"Our focus is on the crime that occurred, the kidnapping," Hyde said. "If in the course of the investigation we have information that will assist him in that, that will be an added benefit."

The reinvestigation of such a crime 49 years later is rare, she conceded.

"There are always cold cases," she said. But when talking about reopening a 1964 baby abduction case, she remarked, "It's unusual. It's certainly unusual."

Fronczak, who lives in Henderson, Nevada, with his wife, Michelle, and daughter, Emma, said his name was Scott McKinley when he was found abandoned in 1965 at age 14 months.

The people who became his parents, Dora and Chester Fronczak of the Chicago area, were able to take possession of him in June 1966 under the belief that the boy was indeed their child, KLAS reported. The Fronczaks adopted and renamed the boy, taking him into their Chicago home.

But as he grew older, Fronczak developed a hunch he didn't fit into the family. He didn't look like his Polish and Croatian parents, who didn't reveal much about the kidnapping, he said.

Last year, Fronczak bought a home DNA kit and took a swabbing from his parents while they were visiting from Chicago. A week later, the results of the test ruled out his being the biological son of Dora and Chester Fronczak, he said.

The DNA technician told him that "there is no remote way that you are the Fronczaks' baby," he said. "I thought, 'Wow.'"

He revealed his story to KLAS just before what was supposed to be his 49th birthday in April. At the time, he said, he didn't even tell his adoptive parents what he learned because it was too agonizing.

The Chicago office of the FBI is now investigating the dual mystery of the kidnapped Fronczak baby and the abandoned Scott McKinley, with assistance from the agency's Las Vegas office, the TV station said. The original kidnapper in the Fronczak case was never found.

Neither Fronczak nor the FBI returned CNN's calls for comment Thursday.

Since obtaining the DNA findings, Fronczak submitted the results to Ancestry.com, a genealogy tracing company. Based on his DNA, Fronczak is discovering blood relatives: he's found a third cousin, KLAS reported.