SEOUL, South Korea — It’s the most heavily fortified border in the world.
The 2.5-mile Demilitarized Zone separating South and North Korea is guarded by tall barbed-wire fences, minefields, sensors and nearly two million troops on both sides.
But one man got through because of loose screws.
When the North Korean man, a former gymnast in his late 20s, crawled over the fence on the southern edge of the DMZ this month, he got past sensors set to trigger alarms to alert South Korean guards.
It was the South Korean military’s most embarrassing breach of border security in years. It raised a disturbing question: How could the man have defected undetected?
This week, the South Korean military said it had solved the mystery: The sensors had loose screws that made the system malfunction. There were no indications that the screws had been deliberately tampered with.
The sensors, installed on the fence in 2015, were supposed to trigger alarms when they detected signs of intrusion, but they failed, military investigators told South Korean reporters during a briefing near the scene on Wednesday.
After the discovery, South Korean officials began checking all sensors along the 255-mile border with the North.
Border guards in the South first found signs on Nov. 3 that an intruder from the North had crawled over the barbed-wire fence on the eastern sector of the border. Military units went on alert, beginning an extensive manhunt.
By the time the North Korean was captured without incident the next day, he was half a mile south of the fence. He told South Korean officials that he wanted to defect. Officials have not released further details, such as his name or his motive for defecting.
Defections across the DMZ are relatively rare and dangerous. When someone from the North crosses the land border undetected, it raises questions about national security in South Korea. The two Koreas have remained technically at war since the 1950-53 Korean conflict was halted in an uneasy truce.
More than 33,000 people from North Korea have defected to South Korea since a devastating famine struck the North in the 1990s. But most have done so through China, which borders the North, eventually making their way to a South Korean embassy in another country.
In November 2017, a North Korean soldier dashed through a hail of bullets fired by his fellow troops to enter the South through Panmunjom, the so-called truce village that straddles the border.
South Korea installed the sensors along the border as part of its efforts to tighten security after another embarrassing border breach.
In 2012, a defecting North Korean soldier not only scaled barbed-wire fences along the border without being detected, but had to knock on the barracks doors of soldiers in the South to get attention.