A person has spoken to the BBC's US partner CBS News about what they saw when the US national crossed.
The unnamed witness, who was part of the same tour group, says the man "gave out a loud 'ha ha ha'" before he ran in between some buildings in the direction of the North Korean border.
"I thought it was a bad joke at first, but when he didn't come back, I realised it wasn't a joke, and then everybody reacted and things got crazy."