A North Korean who was taken into custody Tuesday is believed to have swum across the eastern maritime border, the military said Wednesday.

The military failed to capture him until it spotted him walking along the road on the southern side of the Demilitarized Zone separating the two Koreas.

The man, who was captured after a six-hour chase on Tuesday morning, reportedly wore a wet suit and flippers, which the military found on the eastern shore in the northernmost region of Gangwon Province. He is believed to have cut through a drainpipe below the barbed-wire fences along the frontier beaches.

This is the second time since November that the military has faced embarrassment over its lax security. At that time a former North Korean gymnast jumped over the fence along the inter-Korean border, and loose screws on the sensors prevented an alarm system from activating and immediately alerting the military.

“We picked him up several times on our surveillance cameras since his coming ashore. But no appropriate action was taken and structures inside the drainpipe to block infiltrators look damaged,” the military said. He was first seen at 1:20 a.m. but was brought in six hours later for questioning.

When he was captured, the North Korean was walking in the so-called no-civilian zone south of the DMZ, where civilians are banned from entering without special permits.

The North Korean is believed to be a civilian in his early 20s seeking to defect here, according to authorities, but questions linger over how a civilian with no military training could have made it to the South by swimming in freezing water and cutting through the drainpipe undetected.

The military is facing mounting criticism over repeated security lapses in the same region, where North Korean defectors have made a run for it three times all unnoticed since 2012.

The military vowed to beef up border security in November, when it saw a second breach in the same region in Goseong County, the northernmost part of Gangwon Province. At the time a senior military officer admitted that the military cannot take a look every time something suspicious surfaces.

“We take the latest incident very seriously and we are looking into the event as we speak. We will roll out strict measures once we’re done with the finding,” South Korea’s Joint Chiefs of Staff said.

By Choi Si-young (siyoungchoi@heraldcorp.com)