A non-commissioned officer was found dead on Thursday with an apparently self-inflicted bullet wound in his head at a Marine Corps unit on a western border island near North Korea, military investigators said.

The body of the 23-year-old staff sergeant, whose name was withheld, was discovered about 5:43 a.m. near the unit's gym by his comrade, who left the barracks to patrol the coast of Yeonpyeong Island.

When the comrade found the soldier dead, the staff sergeant had a K-1 rifle in his left hand that was registered to another soldier.

The dead soldier had been on night duty in the unit's situation room since 1:00 a.m. the same day and was supposed to make a shift change around 8:00 to 9:00 a.m.

According to footage from a CCTV camera located at the entrance of the situation room, the soldier exited it with a K-1 rifle and a box of empty cartridges.

A Marine officer said the soldier "is believed to have taken and used somebody else's rifle because he had a key to the storage box for rifles and cartridges during his duty in the situation room."

The soldier joined the Marine Corps in March 2013 before being transfered to the unit on the island in July the same year.

Grief-stricken family members came to the unit by ferry and identified his body.

Military investigators are not ruling out the possibility of a suicide given that he had a weapon in his hand. The motive of his death is unclear. "His postings on social networking service platforms, which we checked out, gave us no clues to the incident," an investigator said. "We plan to meet with his comrades for the investigation." (Yonhap)