JSA

Before the incident, tourists seeking to brush up close to the authoritarian reclusive North regularly visited the JSA - a cluster of buildings that has hosted inter-Korean talks and where troops from both sides stand almost face to face.

Not all DMZ tours stop at the JSA, which is the only spot where visitors can step up to and even briefly over the border into the North.

King's case would be the first successful border crossing by a JSA tourist ever, Zwetsloot said.

For tour agencies, the JSA tour is one of the packages that sell with the highest margin in South Korea with "absolutely no cost involved to the tour companies, except for the bus and the driver," said Jacco Zwetsloot, a former JSA tour guide and now host of the NK News Podcast about North Korea.

Visiting JSA is free of charge for South Korean nationals, but the tour King was on started at $180, according to a Tripadvisor listing.