Private 2nd Class Travis King, the U.S. soldier who crossed into North Korea "willfully and without authorization" on Tuesday, according to U.S. officials, made his move right as he was to be transferred back to the U.S. and dismissed from the military.

U.S. officials said Tuesday that King, 23, was believed to be in the custody of North Korean forces. North Korea's government has not said anything about King since he was apparently detained. 

What motivated the American soldier to run across the border into an isolated, authoritarian country considered one of the biggest threats to peace on the globe remains unclear, but below is what we know so far from statements made by military officials, witnesses and King's family members in the U.S.

How did King cross the DMZ into North Korea?

King crossed over the border in one of the few locations where it would have been possible without the risk of being quickly shot or blown up. The Demilitarized Zone that separates North and South Korea is one of the most heavily fortified and militarized borders on the planet. The vast majority of it is marked with multiple lines of barbed wire and guarded over by heavily armed North and South Korean soldiers on either side.

But in the Joint Security Area (JSA) in the "peace village" of Panmunjom, the weapons and barbed wire give way for a short distance to a simple line on the ground — the Military Demarcation Line. Soldiers from both sides have long guarded the line, within inches of each other, though North Korean troops pulled farther back onto their side during the COVID-19 pandemic and haven't returned to the symbolic standoff.

King was escorted to an airport in Incheon, near South Korea's capital of Seoul, for a flight back to the U.S. to be "separated" from the Army, U.S. officials told CBS News, but after parting ways with his escort at customs, he didn't board the plane. After going through airport security, he somehow returned and managed to join a civilian tour group heading from Seoul to Panmunjom.

CBS News' British partner network BBC News spoke with a man who used to work for a company that ran tours to the JSA for U.S. troops serving in South Korea.

Now the host of the North Korea-themed NK News Podcast, Jacco Zwetsloot told the BBC there was "no way this person could escape from the airport one day and book on to one of these tours the next."

He said it generally takes three days for someone to be authorized to go on one of the trips, and his former clients would have to submit passport and military ID information to U.N. Command, which operates the JSA, in advance.

"When I was leading the tours, we had to change the turnaround time from 48 to 72 hours because there were too many mistakes," he told the BBC, adding that it had become even harder to join the tours since the pandemic. He said to book a spot on one of the limited tours now running would have required research and planning.

A witness who was in King's tour group told CBS News on Tuesday that the American abruptly left the others, laughed, and then ran across the Military Demarcation Line before anyone could act to stop him.

King's brief history with the U.S. military

King has served in the U.S. Army since January 2021, Army spokesperson Bryce Dubee told CBS News. He had not been deployed for active duty, but was sent to South Korea as part of the Pentagon's regular Korean Force Rotation, assigned to 6th Squadron, 1st Cavalry Regiment, 1st Brigade Combat Team, 1st Armored Division.

Dubee said King had received the National Defense Service Medal, the Korean Defense Service Medal, and the Overseas Service Ribbon.

U.S. officials told CBS News that King had served time at a detention facility in South Korea and was handed over to officials at U.S. Army Garrison Humphreys, the hub for U.S. forces in the country, only about a week ago.

He had spent about two months in a South Korean jail after an arrest on assault charges, a South Korean official told the Agence France-Presse news agency. According to South Korean media, he was accused of kicking the door of a police patrol car and shouting expletives at Korean officers.

"I'm absolutely foremost concerned about the welfare of our troop, and so we will remain focused on this," Defense Secretary Lloyd Austin said Tuesday at the Pentagon, without naming King specifically.

What King's family is saying

King's mother told ABC News on Tuesday that she was shocked when she heard her son had crossed into North Korea.

"I can't see Travis doing anything like that," Claudine Gates, from Wisconsin, told ABC.

She said the last time she heard from her son was "a few days ago," and that she just wants "him to come home."

The Daily Beast quoted King's uncle Carl Gates on Wednesday as saying the young soldier had been "breaking down" emotionally over the death of Gates' 7-year-old son, King's cousin, earlier this year.

"His mom came down on a few occasions, and she then talked to him and let him know what was going on with my son. And it seemed like he was breaking down. It affected Travis a lot," Gates told The Daily Beast. "Because he couldn't be here. He was in the Army, overseas."

The news outlet said Gates' young son died in February after a prolonged hospitalization for an untreatable genetic disorder.

"When my son was on life support, and when my son passed away… Travis started [being] reckless [and] crazy when he knew my son was about to die," Gates told The Daily Beast. "I know it was related to what he did."

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• Kim Jong Un
• South Korea
• North Korea

Tucker Reals

Tucker Reals is the CBSNews.com foreign editor, based at the CBS News London bureau.