A North Korean soldier early this month managed to cross the Demilitarized Zone, one of the world’s most heavily guarded borders, by walking, Korea JoongAng Daily reports.

The South Korean military came under criticism after the North Korean defector went undetected until he knocked on the soldier’s barracks 10 meters away from the fence, showing how penetrable the border can become. The unit attempted to cover up the incident which happened under lax security.

According to investigations, the defector managed to go past two barbed wire fences, one electrified, on the North’s side before crossing the border. He climbed over the South Korean barbed fence which stands four meters-high. The soldier then knocked on the barracks of South Korean soldiers and was taken into custody after expressing his will to defect.

The investigation team of the Joint Chiefs of Staff and the National Intelligence Service launched a joint probe on North Korean soldier’s exact route and into how he managed to go through the fences. Investigation on whether the unit was following proper security conditions are also underway.

A third North Korean soldier defected to the South four days after the “walk in”. The soldier claimed he shot two of his superior officers before crossing the border. This comes as tensions between the two Koreas picked up after the intrusion of North Korean boats across the Northern Limit Line and nuclear arms threat.