Sarah Leslie thought she was witnessing a stunt when she saw an American soldier start sprinting toward North Korea.

Leslie and her father, tourists from New Zealand, were part of a group that left Tuesday morning from Seoul to visit the Demilitarized Zone that divides South and North Korea.

Pvt. 2nd Class Travis King was among the group of 43 tourists, Leslie told The Associated Press, although he was casually dressed in jeans and a T-shirt and she had no idea at the time that he was a soldier, or in legal trouble.

King, 23, was a cavalry scout with the 1st Armored Division who had served nearly two months in a South Korean prison for assault. He was released on July 10 and was supposed to travel home Monday to Fort Bliss, Texas, where he could have faced additional military discipline and discharge from the service.

Leslie said her tour group went a step further than many by visiting the Joint Security Area in the village of Panmunjom, allowing tourists to effectively step on North Korean soil inside one of the buildings, which are jointly held. To get on such a tour, she said, required submitting their passports and getting permits in advance.

The group left Seoul by bus in the early morning, and Leslie noticed that King was traveling alone and didn't seem to talk to others on the tour. At one point, she said, he bought a DMZ hat from a gift shop.

The tour was nearing its end Tuesday afternoon — the group had just walked out of the building and were milling about taking photos — when she saw King running “really fast.”

“I assumed initially he had a mate filming him in some kind of really stupid prank or stunt, like a TikTok, the most stupid thing you could do,” Leslie said. “But then I heard one of the soldiers shout, ‘Get that guy.’"

Leslie said the command was shouted by an American soldier, one of a group that patrols the area along with South Korean troops.

But the soldiers didn't have time to respond. She said that after running about 30 feet down a narrow passageway between the distinctive blue buildings, King was over the border and then disappeared from sight. It was all over in a few seconds.

Leslie said she didn't see any people on the North Korean side. The tour group had been told earlier the North Koreans there had been lying low since the outbreak of the COVID-19 pandemic.

After King ran, she said, the soldiers hustled all the tourists into a building and then took them to an information center to give statements. She said many of the tourists, including her father, hadn't seen King run but a soldier explained the events to them.

“People couldn't really quite believe what had happened,” Leslie said. “Quite a few were really shocked. Once we got on the bus and got out of there we were all kind of staring at each other."

Leslie, a lawyer from New Zealand's capital, Wellington, said she'd long had an interest in the Koreas after studying politics at university and seeing South Korean movies.

She said she found it hard to understand why King would head to North Korea.

“I just didn’t think anyone would ever want to do that,” she said.

North Korea stayed silent Wednesday about the detention of the American soldier. 

King is the first known American held in North Korea in nearly five years. Each detention has set off complicated diplomatic wrangling, and this one comes at a time of heightened animosity. On Wednesday, North Korea test-fired two ballistic missiles into the sea in an apparent protest at the deployment of a U.S. nuclear-armed submarine in South Korea for the first time in decades.

“It’s likely that North Korea will use the soldier for propaganda purposes in the short term and then as a bargaining chip in the mid- to long term,” said Yang Moo-jin, president of the University of North Korean Studies in South Korea.

King’s mother told ABC News she was shocked when she heard her son had crossed into North Korea.

“I can’t see Travis doing anything like that,” Claudine Gates, of Racine, Wisconsin, said.

Gates said said she last heard from her son “a few days ago,” when he told her he would return.

White House press secretary Karine Jean-Pierre said that the U.S. government was working with its North Korean counterparts to “resolve this incident.” The American-led U.N. Command said Tuesday the U.S. soldier was believed to be in North Korean custody.

“We’re closely monitoring and investigating the situation,” U.S. Defense Secretary Lloyd Austin told a Pentagon news conference, noting he was foremost concerned about the soldier’s well-being.