South Korea’s military suspects a man who crossed the heavily fortified border into the North on New Year’s Day previously defected to the South.
Officials believe he is a gymnast who jumped the barbed wire fence into South Korea in 2020.
It is not clear why he made the perilous return crossing, or if he is alive or dead.
North Korea has implemented a shoot-on-sight policy at the border to prevent the spread of Covid-19.
Thousands of North Koreans have resettled in South Korea, but crossings through the Demilitarised Zone (DMZ), which separates the two countries, are rare. Most make their way via China.
Instances of defections from South to North are rarer still, with only a handful of cases recorded in recent years.
South Korean troops were sent to investigate on New Year’s Day after surveillance cameras issued an alert that someone was roaming within the military border zone at a point on the east coast.
They could not find the man but discovered he had crossed into North Korea several hours later.
Citing surveillance footage, a high-level military official confirmed that it was indeed the same man who had earlier crossed from North to South Korea in November 2020, using a similar route.
The man, believed to be in his 30s, worked as a cleaner in South Korea, the Yonhap news agency reported. At the time he was detained, he said he was a North Korean gymnast. Other details about him were not immediately known.
“Footage showed he had a identical look and dress as the person who defected from the North in 2020,” a defence ministry official told reporters.
Military chiefs in Seoul said at the weekend they did not know if the person observed crossing was still alive but had sent a message to the North asking for them to be protected.
North Korea acknowledged receiving the message, but there are no details about the man’s fate or his possible motives for returning. An investigation is under way into how he could have made his crossing undetected.
The border between North and South Korea is filled with landmines and surrounded by electric and barbed wire fencing. Surveillance cameras and armed guards are supposed to be on alert 24 hours a day.
Defence officials in Seoul had pledged to overhaul the border defence system after similar breaches in the past.