The investigation detected some problems in maritime and coastal surveillance operations, but the panel is yet to make a conclusion on if there were any false reports or attempts to cover up the case, they added.
The panel involving 30 officials and experts launched the probe on June 20 after revelations that the small wooden boat carrying four North Koreans was found at the port of Samcheok, Gangwon Province, by a South Korean civilian five days earlier.
Though it crossed the inter-Korean maritime border in the East Sea and had sailed for about 57 hours in South Korean waters before docking at the quay, neither the military nor the Coast Guard spotted it.
The Army has coastal surveillance radar systems and thermal observation devices set up in the East Sea for maritime and coastal security, while the Navy operates vessels and P-3C maritime surveillance aircraft.
According to the sources, the investigation found that soldiers were on duty "normally," but their operations to spot objects and read data weren't conducted appropriately.
Soldiers in charge of monitoring radar misunderstood what was picked up by their equipment as "a reflective wave." Though another video monitoring system captured the wooden boat twice, the soldiers pegged the vessel as a South Korean one, the sources said, adding that the aircraft failed to spot its entry.
The panel also found some drawbacks in the military's reporting structure, according to the sources.
Though the civilian alerted local police of the case at 6:50 a.m, the military officers in charge of the security of the region arrived at the scene 45 minutes later.
The panel, however, has yet to draw conclusions on whether the military tried to conceal a failure to detect the boat.
Though the military were well aware of the fact that the four North Koreans arrived at the port unhindered with an intention to defect and they actually docked at the quay, the Joint Chiefs of Staff first said that the boat was found in the vicinity of the port.
Suspicions have mounted over some instructions from the presidential office to minimize the case out of fear that the issue may negatively affect inter-Korean ties.
"The probe has been carried out thoroughly and transparently, and we will announce the probe results at an appropriate time," defense ministry spokesperson Choi Hyun-soo said. (Yonhap)