Allen Thomas was serving in South Korea when he met a woman named Connie, and the two of them had twins—a boy and a girl. It was 1966, and when he was with them, he had nothing but fond memories.

A year later, the laws allowed him to marry Connie and become the legal father of the twins, and he adopted Connie’s son from a previous relationship, Jae-Im, as well.

Then the war ended, and Thomas was set to return to the United States—but his family wasn’t yet able to follow.

The twins were American citizens and had passports, but Connie and Jae-Im were not; Thomas went on ahead and figured he could solve all this in time and bring his family home.

Connie would ask for money, and he would send it, but time dragged on and Thomas couldn’t stand being away from his family. He volunteered to fight in the Vietnam War in hopes that he could visit her during his 30 days of leave.

But it was a bloody war, and he wasn’t back in South Korea until over another year.

“A lot of things had changed,” Thomas said. The relationship wasn’t the same; he and Connie were strained, and though the family sent him off when his 30 days of leave were up, it was one of the most difficult goodbyes.

He didn’t yet know that was the last time he would see them.

Over the next few months, he kept sending letters and war bonds—but then eventually the mail started bouncing.

Then he got a letter from Connie, who said she would give him the twins if he would come to South Korea to get them.

Thomas was bankrupt, and unable to do so at the time. He wrote back—but never heard back from Connie again. The twins were 7 at the time, and he’d lost all contact.

Years later, Thomas moved on and married a woman from his hometown, and started a family, but his twins were still in the back of his mind all these years. How could a father just forget his children?

“My mom had helped my dad search for them for so long,” said Thomas’s daughter. “They did everything they could with the resources that they had, but always seemed to run into roadblocks.”

But Thomas couldn’t give up—he needed these twins to know “that I hadn’t abandoned them.”

The roadblocks continued for 40 whole years, until he made one more desperate plea on Facebook, and it went viral.

“Searching for my twins…” Thomas wrote. He poured out his story on social media, and people, intrigued and empathetic, shared it around the world.

Pamela Slaton, an investigative genealogist, was one of those who read it.

Slaton was adopted as a child herself, and when she began searching for her parents as a young woman she had few resources. Now she has successfully solved adoption cases from all over the world, reuniting families.

Over the years, Thomas had learned that Connie gave up the twins for adoption—like over a thousand other children of American GIs from during the war. The mixed-race children would have been impossible for many of these mothers to raise in Korea, where they would have been disowned by their families.

Thomas also received a letter from the State Department, that said the twins—Sandra and James—had been adopted together in the United States.

He was also told by the government that he had no legal right to the twins anymore.

That was in the ’80s, and Thomas’s search over the years yielded no new clues.

He was stumped, but after his viral Facebook post, others had ideas.

Keonsu Lee, a policeman in Seoul, South Korea, specialized in missing persons and holds the world record for people found. He got in touch with Thomas via Facebook after the post went viral, and Slaton and Lee, and ABC News, teamed up to find these twins.

ABC News’s Korean bureau chief Joohee Cho got in touch with Lee, hoping to unravel this mystery.

The two of them found “Connie” in the Korean government database—a woman who gave birth to twins and married an Allen Thomas—but she passed away in 2007.

But when Cho questioned a woman from the adoption agency and told her about Connie’s story, “I got the feeling she knew who I was talking about but wouldn’t admit it.”

Or at least, couldn’t.

The woman also told Cho that Thomas had no right to these records—he was not family—only siblings had that right.

Connie had another child, Jae-Im, older brother to the twins.

So Cho tracked him down, and found that he was now married with a family of his own, living in Korea.

But this wasn’t a quick fix—Jae-Im didn’t exactly want anything to do with this family, who had for the majority of his life not been part of it.

He remembered as a child that one day he came home from school and the twins were already gone, and he just accepted his mother’s vague explanation that they’d gone somewhere else.

After some conversations with Cho, he relented and agreed to help.

Not long after, Slaton received an email with adoption records from Korea. She learned that the twins’ names and date of birth had been changed, making them near impossible to trace.

She had been searching with the wrong information this whole time.

These twins’ birth records were sealed in the United States as well, but with a lot of persistence, Slaton was able to get the agency to give her clues as to these siblings’ new names. James kept part of his name, and Sandra’s was completely changed but something similar.

With those clues, Slaton tackled her databases again.

“Then this one person, ‘Timothy James Parker’, catches my eye,” she said. Slaton was more than on the right track—it was him, and this revelation would lead to finding his sister as well.

They were both alive, both in the United States, and then she obtained a phone number.

Conversation by conversation, Slaton gave them parts of Thomas’s story, so they knew he was out there, and they were both interested in reconnecting.

But for Thomas, who had searched in vain for nearly half a century, he nearly couldn’t believe the news that his long-lost children were finally found.

He was absolutely stunned into silence.

“They both want to talk to you,” Slaton said. He couldn’t believe it.

She continued, telling Thomas what his son had thought about the search: “He doesn’t know that you’re searching for him yet but is expecting a call today.”

“I said, ‘Did you ever think about finding anyone in your birth family?’ and he said to me, ‘I didn’t think it was possible,'” Slaton recalled. “And he said, ‘Quite honestly, I didn’t think anyone ever cared enough about me to look for me.'”

That brought Thomas to tears.

“That’s so messed up—that they felt abandoned,” he said.

“But Dad, you didn’t abandon him—and now he gets to know that,” his daughter Charlene told him.

Sandra was now Susan, and was adopted by a wonderful woman who adopted six other children to give them a home. Neither she nor her brother Tim ever heard from their mother again after they were adopted, even though they tried writing.

But they always wanted to ask: Why?

They didn’t have any photos of Thomas, and had forgotten he even cared.

Now Susan, Tim, and their biological father Thomas all had expressed that they wanted to reach out to each other. So Slaton made the call.

“Tim, I am with your biological father,” she said, with Thomas sitting right beside her.

“Biological father?!” Tim exclaimed. Slaton tried to continue, but he was too surprised. “Are you telling me he’s still alive?”

“I have a father!” Tim said.

“I can’t believe this is real,” Susan said between tears. “I’m an all grown woman and here I am acting like a baby, crying.”

The emotional reunion after 40 years was overwhelming for all three parties—and then they agreed to meet in person.

“I just can’t believe you’re my actual father. I’m standing right here looking at you,” Tim told his father during their reunion. “You don’t know what I’m feeling—I’m going through right now.”

“I’ve been looking for a long time,” Thomas told them.

Watch the investigation and the emotional reunion below: