'He was going so fast': US officials try to piece together why soldier bolted into North Korea

Just one day before crossing into North Korea, Private Travis King texted his US military handlers to let them know he had arrived at his gate at Incheon Airport in Seoul and was preparing to board a plane back to the US.

King, a junior enlisted soldier assigned to US Forces Korea, had faced assault charges in South Korea and was due to be removed from the US military upon his return to Fort Bliss, Texas.

But while he cleared customs, he did not get on the plane as scheduled on Monday, US officials told CNN. His escorts could not accompany him all the way to the gate to verify that he had boarded. Instead, on Tuesday, he booked a tour with a private company of the Joint Security Area inside the demilitarized zone, which divides North and South Korea.

As tourists milled about taking photos, King inexplicably made a run for it across the demarcation line into North Korea, an eyewitness on the same tour and US officials familiar with the case told CNN. Unlike most of the heavily fortified zone, which is 160 miles long and 2.5 miles wide, the actual border line between North and South Korea inside the JSA does not have a physical barrier.

King at first tried to enter Panmungak Hall, which is a North Korean facility in the Joint Security Area, one of the US officials said. But the front door was locked - so he ran to the back of the building, at which point he was hurried into a van and driven away by North Korean guards, the official said.

"Get him!" a soldier on the South Korean side yelled, according to the eyewitness, Sarah Leslie. But it was too late.

"He was going so fast, and we were so close to the border, that he was gone by then," Leslie added.