SEOUL, South Korea — A North Korean man wearing a wet suit and flippers crossed the eastern maritime border with South Korea this week, military officials said on Wednesday. The South’s soldiers failed to detect him until he was walking down a road south of the heavily guarded frontier.

The crossing marked the second embarrassing breach for the South Korean military’s border security in recent months. In November, another North Korean man, a former gymnast, crawled over the border fence and was not captured until he was half a mile south of the border. The military later said that sensors supposed to trigger alarms that alert South Korean guards malfunctioned because of loose bolts.

The latest infiltrator from the North swam across the border on Tuesday, coming ashore south of the 2.5-mile-wide Demilitarized Zone, or DMZ, the no-man’s buffer zone that separated the two Koreas after the war, the South Korean military said in a statement on Wednesday.

Officials were investigating the man’s motive in crossing the border, and said he may be a defector from the North. He came ashore by crawling through a drain beneath a barbed-wire fence that South Korea erected along the frontier beaches to deter North Korean infiltrators.

A closed-circuit television camera at a military checkpoint first captured him walking down a road to the south at 4:20 a.m. Tuesday, but it was not until three hours later that soldiers captured him for interrogation. When he was captured, the man was in the so-called civilian-control zone south of the DMZ, where no civilians are allowed to travel without military permit.

“Our military did not take appropriate actions, although the man had been detected in its surveillance system several times since coming ashore,” the military said.

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 armistice.

The Korean Demilitarized Zone is one of the world’s most heavily armed frontiers, guarded by tall barbed-wire fences, minefields, sensors and nearly two million troops on both sides.

Defections across the DMZ are relatively rare and dangerous. 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.

More than 33,000 people from North Korea have defected to South Korea since a devastating famine struck the country 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.