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When American soldier Allen Thomas waved goodbye to his twins at an airport in Korea in 1971, he had no idea it would be the last time he would see them.

More than 40 years later, the veteran has turned to social media in an effort to locate his long-lost children, who were put up for adoption after he returned to the United States.

"I just want to see my kids and know that they are okay. I want to let them know I love them," Thomas said.

"I want to share our family health history with them, because some of it is serious. I want them to know that I have never stopped trying to get them."

A Facebook post by Thomas has been shared 1.2 million times in a few days, and an online group set up to generate leads has garnered 25,000 members.

The quest has left Thomas so emotional that he has difficulty even speaking about the situation and asked his adult daughter, Charlene Roberts, to share details of the family history.

"I want the kids to see it and I want them to know he's been looking for them," Roberts told NBC News on Tuesday.

Thomas was just 18 years old when he joined the Army, and he shipped out to Korea the next year. That was where he met a woman who was soon pregnant with twins. A boy named James and a girl named Sandia were born Sept. 10, 1967, and he and their mother married some months later.

Allen Thomas in his uniform. Allen Thomas

The plan was that the young family would eventually return to the United States, and Thomas has the documents to show he registered the kids with the State Department and applied for passports.

But before his wife's paperwork was ready, the marriage "disintegrated," Roberts said.

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When Thomas' Korea tour was up, he stayed in Asia to be close to the kids, she said. In January 1971, he took a 30-day leave and traveled from Vietnam to Korea for a visit.

"That was when he last saw his kids," Roberts said.

The Army wanted him back in the states, but his estranged wife didn't want to come and wouldn't let the children leave, Roberts said. "He even considered going AWOL to take his kids," she said.

Instead, he came back with the idea that he would attempt to arrange a reunion from the United States.

Allen Thomas is searching for his long-lost twins, James and Sandia, who were born in Korea in 1967. Their mother put them up for adoption in 1976 without his knowledge. Allen Thomas

He sent letters and money, Roberts said, but eventually the lines of communication were cut off. His correspondence was returned to him unopened, and he obtained a default divorce, she said.

Back home in New Hampshire, Thomas began a romance with a childhood friend, and they got married in 1973. He then adopted her two children from a previous relationship — Roberts and her brother — before going on to have a third child.

In 1974, the mother of the twins contacted him one last time with an offer to relinquish Sandia and James, who were also called Sandra and Jamie.

But at the time, Thomas and his second wife were going through bankruptcy and could not come up with the money to bring them over, despite appeals to the Army and Red Cross, according to a 1980 letter to his local congressman.

By the time the couple got back on their feet financially, it was too late.

The couple had written to the State Department and elected officials, asking for assistance. They also reached out to the Pearl S. Buck Foundation, which was started by Pulitzer-winning "The Good Earth" author to assist children overseas.

In 1980, the foundation sent a letter with the findings of its investigation: The children had been adopted by Americans four years earlier, right after Thomas left the Army, which meant his permission was not needed under Korean law.

"Mother [natural] will not give address of children," the letter read.

A letter from the Pear S. Buck Foundation to Allen Thomas. Allen Thomaa

The trail had run cold but the Thomases still didn't give up the hunt, registering with Korean-American groups where the twins might be members.

"We have documentation to show that he never stopped searching for his kids," Roberts said. "But there is no trace of them."

Thomas was widowed about 10 years ago, and the retired factory worker recently moved from Colorado to live with Roberts and her husband in Mossyrock, Washington, about two hours from Seattle.

She started helping him use the Internet to reignite his search for the twins, but there was no progress. Then they tried Facebook "and it's gone into turbo-mode," Roberts said.

The attention is a double-edged sword: It's increased the chances that Thomas might actually find James and Sandia, but it's also dredged up a lot of emotion.

"It's just gut-wrenching," his daughter said. "My dad doesn't cry, but this has ripped his heart out. It's very difficult."

In the statement he provided to NBC News, Thomas said that if the children don't want to see him, he understands and "would respect that."

"I just need to know they're OK and tell them I love them. These are my kids, and I've never stopped loving them," he said.