South Korea's military says it has arrested a Marine private for helping a colleague who killed four other Marines in a shooting rampage on Monday.

An official with the Defense Ministry says the private, identified only as Jung, is suspected of helping his friend steal ammunition from a weapons storage area shortly before he opened fire at a barracks on Gangwha Island, located near the tense maritime border with North Korea.

The shooter, a 19-year-old corporal identified only as Kim, has been hospitalized with wounds from a grenade blast in an apparent suicide attempt.

The defense official says Jung admitted that he and Kim had hatched a plan to kill their colleagues and desert from their Marine unit to escape the abuse they had suffered at the hands of their superiors. Jung denies taking part in the shooting, but the official says Kim has implicated his friend.

The military previously said Kim had shown signs of mental illness before the incident, and left behind a note lamenting his life, describing himself as a “troublemaker.” He also reeked of alcohol before the shooting, suggesting he may have been intoxicated.

The four slain Marines were memorialized Wednesday at a joint funeral. In his eulogy, Lieutenant General Yoo Nak-jun, the Marine Corps commander, said the corps will try to ensure “our soldiers never again hurt each other.”

The shooting rampage was the worst in the South Korean military since 2005, when eight soldiers were killed by a soldier who threw a grenade and opened fire at a front line army outpost.

The incidents have raised questions about the level of discipline in the South's 650,000-strong military force, which is largely made up of young men who are required to serve two years. Critics have said the men are abused and harassed.